Laser light source

ABSTRACT

The invention provides a compact laser light source whose wavelength can be designed freely in a wavelength band in which the semiconductor laser has not been put to practical use by combining an efficient nonlinear optical crystal and high-power semiconductor lasers for optical communication. In one embodiment, the laser light source includes: a first laser for generating a laser beam of a wavelength λ 1 ; a second laser for generating a laser beam of a wavelength λ 2 ; and a nonlinear optical crystal that allows the laser beam of wavelength λ 1  and the laser beam of wavelength λ 2  as inputs and outputs a coherent beam having a wavelength λ 3  of a sum frequency that satisfies a relationship of 1/λ 1 +1/λ 2 =1/λ 3 . The wavelength λ 3  of the sum frequency is 589.3±2 nm that is equivalent to the sodium D line.

TECHNICAL FIELD

This invention relates to a laser light source, and more specifically,to a laser light source for outputting a coherent beam of either awavelength in the yellow color range or the sodium D-line wavelengthefficiently using lasers and a nonlinear optical crystal, a laser lightsource capable of tuning a laser beam in the wavelength band of 2-3 μmin the mid-infrared region, and a laser light source for outputting alaser beam of oxygen absorption lines existing at wavelengths of 759 nmto 768 nm.

BACKGROUND ART

Presently, the lasers that have been put into practical use are known toinclude gas lasers, such as the He—Ne laser and the Ar laser, solidstate lasers, such as the Nd—YAG laser, dye lasers, and semiconductorlasers. FIG. 1 shows a relationship between wavelength band and outputpower of lasers. In recent years, compact, lightweight, and inexpensivesemiconductor lasers have become popular in wavelength band 102 in thevisible and infrared regions. Especially, in the optical communicationfiled, 1.3-μm band and 1.5-μm band semiconductor lasers for signal lightsources and the 0.98-μm band and 1.48-μm band semiconductor lasers forfiber amplifier pumping have come into widespread use. Moreover, thesemiconductor laser is used also as lasers for CD and red lasers, andthe semiconductor laser is used also in wavelength band 101 in thevisible and infrared regions used for reading and writing storage media,such as DVD and Blue-ray.

However, the semiconductor laser has not been put into practical use inwavelength band 111 of the green, yellowish green, and yellow ranges ofwavelengths of 0.5-0.6 μm and in wavelength band 112 of the mid-infraredrange 2-5 μm, and hence the gas laser and the solid-state laser, whichare expensive and consume large electric power, are being used.

Optical characteristics, such as refractive index and absorption, ofoptical media of liquids, glasses, etc. have become important evaluationitems to specify characteristics of optical instruments and to controlqualities, such as accuracy and purity, of foods, medicines, etc. Formeasurement of these optical characteristics, the light source forgenerating the sodium D-line of wavelengths of 589-590 nm in the yellowrange included in wavelength band 111 is being used.

For example, a relationship between the refractive index and the sugarcontent in a liquid is defined as Brix value by ICUMSA (InternationalCommission for Uniform Methods of Sugars Analysis) and a method forfinding the sugar content from measurement of the refractive index isprovided. This method is applied to sugar content measurement of fruitsand alcoholic beverage, being used widely industrially.

In the field of medicines, the Japanese pharmacopoeia defines refractiveindices of solutions in which respective medical agents are solved asone of quality control measures of medical agents. There is a case wherea “right-hand-system” medicine that has a spiral structure, such asthalidomide, may have a medicinal effect, but a “left-hand-system”medicine may serve as a poison. It is impossible to separatephysicochemically substances each having mutually inverse spiralstructure like this from each other. However, it is known that thesesubstances exhibit different optical activities, and can easily beidentified optically. Then, after phytotoxicity accidents likethalidomide, the Japanese pharmacopoeia defines a measurement of angleof rotation using the sodium D line. Medicines exhibiting such aproperty include a large number of medicines, such as menthol,prostaglandin, β lactam antibiotics, and quinolone antibacterial agents,besides thalidomide.

Presently, a laser light source for generating the sodium D line has notbeen realized, and a sodium vapor lamp or yellow LED is used as a lightsource. A light beam from a sodium vapor lamp is excellent inmonochromaticity, but is a divergent light beam emitted in alldirections. Therefore, it is difficult to collimate it, and so accuratemeasurement of optical characteristic is difficult. Moreover, sincefocused energy does not reach a high level, it is necessary to use ahigh-power lamp.

On the other hand, the spectral linewidth of the yellow LED is as wideas approximately 20 nm. Because of this, the spectral linewidth isintended to be narrowed by extracting a spectrum near the sodium D lineusing an optical filter, but there is a limit to narrow it. Moreover,since the yellow LED light lacks coherency, there is a limit inimproving measurement accuracy.

In the context of such facts, improvement in accuracy of opticalevaluation methods that have been prescribed with the sodium D-linewavelengths are being demanded in many industrial fields, such asquality control of foods and medicines. If a laser at the sodium D linecan be realized, measurement using light interference will becomepossible. With the use of optical interference, measurement accuracy ofthe refractive index of various liquids and optical media includingfoods and medicines can be improved from the present value by about twoorders of magnitude, and low consumption power and miniaturizationbecome possible as well.

The electronic structure of sodium and characteristics of lightgenerated from its energy transition will be described (see Non-patentdocument 1). It is known that wavelengths of emission from a sodium atomare 589.592 nm (D1 line) and 588.995 nm (D2 line). The D1 line and D2line are collectively called D line, and the wavelength of D line may becalled 589.3 nm, taking an average of the two wavelengths. FIG. 2 showsthe energy levels of a sodium atom. The D line is generated accompanyinga transition from the 3P level, which is the first excitation state, tothe 3S level, which is the ground state. The 3P level has a finestructure of 3P_(1/2) and 3P_(3/2). Emission of the D1 line is caused bya transition from 3P_(1/2) to 3S_(1/2) and emission of the D2 line iscaused by a transition from 3P_(3/2) to 3S_(1/2).

The 3S_(1/2), 3P_(1/2), and 3P_(3/2) levels have hyperfine structuresdue to interaction of the electron magnetic moment and the intrinsicmagnetic moment of the atomic nucleus. The 3S_(1/2) level splits intotwo levels whose energy difference is 7.3 μeV, the 3P_(1/2) level splitsinto two levels whose energy difference is 0.78 μeV, and the 3P_(3/2)level splits into four levels whose energy difference is 0.48 μeV(maximum difference).

In order to realize a laser emitting light at the D1 line wavelength andthe D2 line wavelength, it is necessary to create population inversionsbetween energy levels corresponding to each light. In order to create apopulation inversion, it is necessary to construct a three-level systemor four-level system. However, in the energy levels shown in FIG. 2,relaxation of 3P_(3/2) to 3P_(1/2) is a forbidden transition, and arelaxation time of 3P_(1/2) to 3S_(1/2) is 15.9 ns (Non-patent document2). For example, when comparing it with a relaxation time of 3.2 μs inthe TiAl₂O₃ laser, the former is shorter than the latter by two ordersof magnitude or more. Therefore, it is difficult to create a populationinversion between 3S_(1/2) and 3P_(1/2), so laser oscillation of thesodium D-line wavelength has not yet been realized. Alternately,although laser oscillation using the hyperfine structure is conceivable,the energy differences of the hyperfine structures of the 3S_(1/2),3P_(1/2,) and 3P_(3/2) levels in a sodium atom are about four orders ofmagnitude smaller than energy at room temperature (300K), 25.8 meV.Because of this, excitation at room temperature is distributed to theboth split levels in the hyperfine structure almost equally, and cannotcreate a population inversion. For these reasons, lasers at the sodiumD1 line and D2 line have not been realized until now.

Conventionally, semiconductor lasers have been put to practical use onlyin the wavelength bands of shorter than 500 nm and longer than 620 nm.In the wavelength band of 500-620 nm, solid-state lasers have beenrealized by a second overtone generation method using fiber lasers orthe Nd—YAG laser, but a solid-state laser of an arbitrary wavelength hasnot yet been realized.

On the other hand, the second overtone generation method (SHG method)using a nonlinear crystal is known as a method for generating coherentlight in the visible range. In order to generate light of the D1 line orD2 line by this method, light of the 1179.2-nm wavelength or the1178.0-nm wavelength is required. Unfortunately, although thesewavelengths can be attained by semiconductor lasers, it is extremelydifficult to obtain a laser capable of delivering necessary power.

Visible light can also be obtained by generating sum frequency of twoexcitation laser beams with a nonlinear crystal. In this method, energyof sum frequency light is given by a sum of energies of the twoexcitation beams. This method comes with an advantage that freedom of acombination of wavelengths of the two excitation beams is widenedbecause a desired wavelength is obtained by sum frequency generation.Therefore, it is the most practical method to realize a laser of anarbitrary wavelength. However, generally nonlinear optical phenomena hada problem of low efficiency. In order to solve this problem, selectionof an existing laser device that can deliver high excitation lightintensity and that is compact and consumes low electric power as well asimprovement in characteristics of a nonlinear optical crystal becomeimportant.

The first object of this invention is to provide a laser light sourcethat generates a coherent beam that is energy-efficient with a narrowlinewidth and excellent directivity and generates a coherent beam of awavelength of the sodium D line.

Conventionally, the laser microscope that scans a sample with a confocallaser beam to obtain an optical tomogram is known. The laser microscopeis being used for analyzing distributions of a substance withfluorescent labeling in a tissue and cell. Moreover, there is known aflow cytometer that irradiates a laser beam onto a stream of cellsaligned in a line, and analyzes and isolates a cell preparativelydepending on fluorescence intensity. The flow cytometer is a measuringapparatus that uses a flow cytometry method for identifying a cellqualitatively using properties of a cell, for example, a size, a DNAcontent, etc. as optical parameters.

Although the fluorochrome is used as fluorescent labeling in recentyears, since the fluorochrome was a foreign matter for cells, there areproblems that the properties of a cell is affected, a cell dies, etc.Therefore, the method for performing fluorescent labeling with a greenfluorescent protein extracted from jellyfish etc. is being used.Moreover, fluorescent proteins that exhibit florescence of yellow andred have been obtained by mutation and genetic manipulation of greenfluorescent proteins (for example, see Non-patent document 3), and moredetailed measurement and analysis are being conducted using multicolorfluorescence.

Since the red fluorescent protein has the absorption maximum atwavelengths of 560-590 nm (for example, see Non-patent document 4), alaser light source having an oscillation wavelength in this wavelengthband is expected. Since lasers having oscillation wavelengths in thiswavelength band are only large-sized lasers, such as a dye laser; a532-nm solid-state laser and a 543-nm He—Ne laser are being usedinstead. However, since at these wavelengths, fluorescence wavelengthsof green fluorescent proteins and absorption wavelengths of yellowfluorescent proteins overlap remarkably, these are inconvenient formeasurement and analysis using multi-color fluorescent proteins.

Very recently, there is reported Kindling red fluorescent protein thatemits red fluorescence stably for a long time of more than 72 hours byirradiation of intense green laser beam (wavelengths of 530-560 nm) (forexample, see Non-patent document 5). An effect that the use ofKindling-Red fluorescent protein enables long-time observation of how acell divides using fluorescence and other effects are expected. However,with the conventional 532-nm solid state laser and the 543-nm He—Nelaser, overlap between the fluorescence wavelengths of green fluorescentproteins and the absorption wavelengths of yellow fluorescent proteinsis are significant. Therefore, realization of a compact solid-statelaser having an oscillation wavelength as close to 560 nm as possible isdesired.

Moreover, metalloporphyrin is a molecule contained in a protein thatbears an important function for life activity of animals and plants,such as photosynthesis and respiratory metabolism, having an absorptionmaximum near the 590-nm wavelength. Since these emission wavelengths ofmetalloporphyrin exhibit peaks near 600 nm, if a laser of the 589-nmwavelength is used, overlap with the emission wavelengths is too largeto perform measurement. Consequently, a golden yellow laser of the585.0-nm wavelength is needed.

Furthermore, the 546.1-nm wavelength (yellowish green) corresponding toone of emission lines (e-line) emitted from a mercury vapor lamp is awavelength at which human's visibility is highest, being used as awavelength of the refractive index standard for optical glasses. Asshown in FIG. 1, in the green, yellowish green, yellow ranges of 500-600nm included in wavelength band 111, efficient and highly stable laserlight sources are needed.

However, as described above, semiconductor lasers have been put topractical use only in the wavelength bands of shorter than 500 nm andlonger than 620 nm. In the wavelength band of 500-620 nm, a solid-statelaser of an arbitrary wavelength has not yet been realized. In order togenerate light in the yellow range by the SHG method, a light source ofa wavelength of 1092.2 nm, 1120.0 nm, or 1170.0 nm is needed. However,although the semiconductor lasers can oscillate at these wavelengths, itis very difficult to obtain a laser capable of delivering necessaryoutput.

As described above, in making use of nonlinear optical phenomena, it isvery important to select an existing laser that can deliver highexcitation beam intensity and that is compact and consumes low power aswell as improving the characteristics of a nonlinear optical crystal.

The second object of this invention is to provide a laser light sourcefor generating a coherent beam in the yellow range that has a narrowlinewidth and excellent directivity and is energy-efficient.

From the viewpoint of environmental protection as well as health andsafety, it is strongly desired to establish ultralow volume analyticaltechniques of environmental gases, such as NO_(x), SO_(x), and ammoniasystem, absorption peaks of water, many organic gases, and residualpesticides. As the ultralow-volume analytical techniques, a quantitativeanalysis in which a gas to be measured (measured gas) is adsorbed in aspecific substance and an electrochemical technique is performed, and anoptical method for measuring optical absorption property intrinsic to ameasured substance are common. Among these, the optical method hasfeatures that real-time measurement is possible and a widespread areathrough which measuring light passes can be observed.

Absorption peaks of a measured substance result form vibration modes ofan interatomic bonding, and exist mainly in the mid-infrared region of2-20 μm. However, in wavelength band 112 in the mid-infrared regionshown in FIG. 1, a laser capable of continuous oscillation at roomtemperature has not yet been put to practical use, but only research anddevelopment of the quantum cascade laser is being advanced.Industrially, although the need for mid-infrared light is high, a factthat there is no practical laser light source becomes a largeobstruction to applications.

Since there is no practicable light source in the mid-infrared region,when performing microanalysis of various gases etc. using existingsemiconductor lasers (0.8-2 μm) for communications, absorption at thesecond overtone of the fundamental absorption wavelength (=½ of thefundamental absorption wavelength) and at the third overtone. (=⅓ of thefundamental absorption wavelength) will be used. As far as the secondovertone is concerned, required sensitivity may be obtained. However,measurement at a high-order absorption peak of the third or higherovertone comes with a limit in detection, because the amount ofabsorption itself is small. Therefore, this method will bring decreasein sensitivity by about three orders of magnitude as compared to themeasurement at the original fundamental absorption wavelength.

Therefore, in order to obtain high detection sensitivity in analyzingenvironmental gases, gases involving risk, etc., it is indispensable todevelop a mid-infrared laser light source. In recent years, it wasreported that mid-infrared light was generated in the vicinity of the3-μm wavelength, and an operation as a gas sensor was verified (forexample, see Non-patent document 6). A light source used in a gas sensorgenerates mid-infrared light by difference frequency generation using alithium niobium oxide (LiNbO₃) wavelength converter device that has aperiodically poled structure.

However, the wavelength converter device having the periodically poledstructure generates only mid-infrared light of a single fixedwavelength. Then, in order to make the wavelength variable so thatdifferent kinds of gases can be detected together, several methods areknown as follows. (1) Several periods are provided in a singlewavelength converter device (for example, see Non-patent document 7).(2) Period is changed by means of a structure called Fanout Grating (seethe aforesaid Non-patent document 6). (3) Effective period is changed bymaking an excitation beam incident on the device slantingly (forexample, see Non-patent document 8)

Although these methods can sweep a wavelength in a wide range, since theelement with various periods had to be bundled, there was a problem thatmany operation processes were needed. Moreover, the technique of makingan excitation beam incident on an element slantingly comes with aproblem that it is difficult to create a waveguide structure in thedevice to attain high efficiency.

The third object of this invention is to provide a laser light sourcecapable of tuning a laser beam in the mid-infrared region between 2-μmand 3-μm wavelength.

In recent years, environmental problem is coming to the fore greatly,and especially, attention centers on influences of dioxin on human body.In an incinerator that is one of origins of dioxin, generation of dioxincan be suppressed by controlling the combustion state of the furnace.For monitoring the combustion state, thermometers, CO concentrationmeters, and oximeters are needed.

As one technique to detect gas concentrations, there is known a methodin which measured gases are irradiated with a laser beam and theirabsorption properties are observed. Since each gas has intrinsicabsorption lines, the gas concentration can be detected by scanning alaser beam having a wavelength near the absorption line and observing anabsorption spectrum. Points required for the laser beam at this occasioninclude monochromaticity, i.e., being a single-mode laser beam,delivering an output of a few mW to a few tens mW suited to gasdetection, capability of stable wavelength scanning, long life, etc.

A laser beam used in the oximeter is in wavelength band 113 including aplurality of oxygen absorption lines existing at wavelengths of 759 nmto 768 nm, so gallium arsenide semiconductor lasers are being used (forexample, see Patent document 1). A gallium arsenide semiconductor laseris manufactured by growing semiconductor crystals whose latticeconstants almost agree with the lattice constant of gallium arsenide.

Semiconductor lasers are divided into the edge emitting type laser whosewaveguide is manufactured in parallel to a substrate and the surfaceemission-type laser that emits light perpendicular to a substrate.Regarding gallium-arsenide edge emitting type lasers, relativelyhigh-power single-mode lasers have been developed, but do not havestructures to control their oscillation wavelengths. Consequently, theoscillation wavelength of the gallium-arsenide edge emitting type laseris determined at a point at which a gain peak of the active layer and aresonant mode of the resonator coincide. Therefore, the laser easilyjumps among longitudinal modes at the time of wavelength scanning, andstable wavelength scanning is hard to perform.

As structures for controlling an oscillation wavelength, thedistribution feedback (DFB) type, the distribution Bragg-reflection(DBR) type, etc. are well known. For these structures, it is necessaryto manufacture semiconductor crystal whose refractive index is variedperiodically in a direction parallel to the substrate, namely, whosecomposition is varied, in the semiconductor crystals. A manufacturemethod is that the surface of the semiconductor crystal is etched to aperiodical structure, such as a corrugated shape, and thereon asemiconductor crystal of a different composition is grown. If the laseris intended to oscillate at the 763-nm wavelength in order to detect theoxygen concentration, it is necessary to suppress absorption at thewavelength and crystals of high aluminum concentrations must be used.However, if the aluminum concentration is high, there is a problem thatthe crystal is likely to be oxidized when manufacturing the periodicstructure.

The surface emission-type laser is a kind of the DBR laser. In thesurface emission-type laser, since a direction of emission isperpendicular to the substrate, the laser needs a DBR structure having arefractive index distribution in the perpendicular direction to thesubstrate. That is, it is only necessary to grow semiconductor crystalseach of which is a layer parallel to the substrate and has a differentcomposition so as to form a periodically stacked layers of crystals.Sine the manufacture can be completed with one round of semiconductorcrystal growth, the manufacture is easy. However, since light passesthrough the active layer in a vertical direction in the surfaceemission-type laser, large gain cannot be obtained. In order to obtainsufficient output, a method for increasing the area of emission isconceivable. However, if the area of emission is increased, the laserwill oscillate in a plurality of transverse modes, departing from asingle-mode operation. If emission intensity of an order of mW necessaryfor detection of oxygen concentration is intended to be obtained whilekeeping a single-mode operation with a limited area of emission, currentnecessary for emission will concentrate in a minute area to increase thecurrent density. For this reason, there is a problem that a life of thesurface emission-type laser becomes as short as a few months.

The fourth object of this invention is to provide a laser light sourcethat is high-power and long-life at wavelengths of 759 nm to 768 nm thatare the oxygen absorption lines.

[Patent document 1] Japanese Patent Application Laid-open No.6-194343(1994)

[Patent document2] U.S. Pat. No.5,036,220

[Patent document 3] Japanese Patent Laid-open No. 4-507299(1992)

[Non-patent document 1] K. Kubo and K. Katori, “Spin and Polarization,”BAIFUKAN, p. 21-24 (Oct. 31, 1994)

[Non-patent document 2] Harold J. Metcalf and Peter van der Straten,“Laser Cooling and Trapping,” Springer, pp. 274 (1999)

[Non-patent document 3] G. Patterson et al., J. Cell Sci., No. 114, pp.837-838 (2001)

[Non-patent document 4] A. F. Fradkov et al., Biochem. J., No. 368, pp.17-21 (2002)

[Non-patent document 5] D. M. Chudakov et al., Nat. Biotechnol. No. 21,pp. 191-194 (2003) [Non-patent document 6] D. Richter, et al., AppliedOptics, Vol. 39, 4444 (2000)

[Non-patent document 7] I. B. Zotova et al., Optics Letters, Vol. 28,552 (2003)

[Non-patent document 8] C.-W. Hsu et al., Optics letters, Vol. 26, 1412(2001)

[Non-patent document 9] A. Yariv, “Quantum Electronics,” Third Ed., pp.392-398 (1988)

[Non-patent document 10]http]//laserfocusworld.365media,comilaserfocusworld/search Resultasp?cat=48903/&d=453&st=1

[Non-patent document 11] R. M. Schotland, Proc. third Symp. on RemoteSensing of Environment, 215 (1964)

[Non-patent document 12] IEEE Photonics Technology Letters, Vol. 11, pp.653-655 (1999)

[Non-patent document 13] Proceedings of the 15th Annual Meeting ofInstitute of Electrical and Electronic Engineers, Lasers andElcctro-Optics Society, 2002 (IEOS2002), Vol. 1, pp. 79-80 (2002)

DISCLOSURE OF THE INVENTION

The present invention provides a compact laser light source that allowsa user to design its wavelength freely in a wavelength band in which thesemiconductor lasers have not been put to practical use by combining anefficient nonlinear optical crystal and high-power semiconductor lasersfor optical communication.

In order to achieve the first object, this invention is a laser lightsource that comprises: a first laser for generating a laser beam of awavelength λ₁; a second laser for generating a laser beam of awavelength λ₂; and a nonlinear optical crystal that uses laser beams ofwavelengths λ₂, λ₁ as inputs and delivers a coherent beam of awavelength λ₃ of a sum frequency that satisfies a relationship of1/λ₁+1/λ₂=1/λ₃; wherein the wavelength λ₃ of the sum frequency is awavelength of 589.3±2 nm corresponding to the sodium D line.

Moreover, in order to achieve the second object, this invention is alaser light source that comprises: a first laser for generating a laserbeam of a wavelength λ₁; a second laser for generating a laser beam of awavelength λ₂; and a nonlinear optical crystal that uses laser beams ofwavelengths λ₂, λ₁ as inputs and delivers a coherent beam of awavelength λ₃ of a sum frequency that satisfies a relationship of1/λ₁+1/λ₂=1/λ₃; wherein the wavelength λ₁ is 940±10 nm, the wavelengthλ₂ is 1320±20 nm, and the wavelength λ₃ of the sum frequency is546.1±5.0 nm that corresponds to a yellow range.

Setting the wavelength λ₁ to 980±10 nm and the wavelength λ₂ to 1320±20nm, the wavelength λ₃ of the sum frequency becomes 560.0±5.0 nm thatcorresponds to the yellow range. Alternately, setting the wavelength λ₁to 1064±10 nm and the wavelength λ₂ to 1320±20 nm, the wavelength λ₃ ofthe sum frequency becomes 585.0±5.0 nm that corresponds to the yellowrange. Further alternatively, setting the wavelength λ₁ to 940±10 nm andthe wavelength λ₂ to 1550±30 nm, the wavelength λ₃ of the sum frequencybecomes 585.0±5.0 nm that corresponds to the yellow range.

Furthermore, in order to achieve the third object, this invention is alaser light source that comprises: a first laser for generating a laserbeam of a wavelength λ₁; a second laser for generating a laser beam of awavelength λ₂; and a nonlinear optical crystal that uses the laser beamsof wavelengths λ₂, λ₁ as inputs and delivers a coherent beam of awavelength λ₃ of a difference frequency that satisfies a relationship of1/λ₁−1/λ₂=1/λ₃; wherein the wavelength λ₁ is in a range of 0.9-1.0 μmand the nonlinear optical crystal has a periodically poled structure ofa single period that is configured so that the wavelength λ₃ variesbetween 3.1 μm and 2.0 μm when the wavelength λ₂ varies between 1.3 μmand 1.8 μm.

Even furthermore, in order to achieve the fourth object, the laser lightsource comprises: a distributed feedback semiconductor laser foroscillating a laser beam having a wavelength twice the wavelength of asingle absorption line selected from among oxygen absorption lines thatexist at wavelengths of 759 nm to 768 nm; an optical waveguide having asecond-order nonlinear optical effect; a polarization maintaining fiberthat connects an output of the distributed feedback semiconductor laserand one end of the optical waveguide.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing a relationship between laser wavelength bandand output power;

FIG. 2 is a diagram showing the energy levels of a sodium atom;

FIG. 3 is a block diagram of a laser light source according to oneembodiment of this invention;

FIG. 4 is a diagram showing a relationship of wavelength between anexcitation laser 1 and an excitation laser 2 for obtaining a wavelengthof the sodium D line by sum frequency generation;

FIG. 5 is a block diagram of a laser light source of the sodium D-linewavelength according to Embodiment 1-1 of this invention;

FIG. 6 is a block diagram of a laser light source of the sodium D-linewavelength according to Embodiment 1-2 of this invention;

FIG. 7 is a block diagram of a laser light source of the sodium D-linewavelength according to Embodiment 1-4 of this invention;

FIG. 8 is a block diagram of a laser light source of the sodium D-linewavelength according to Embodiment 1-5 of this invention;

FIG. 9 is a diagram showing a relationship of wavelength between theexcitation laser 1 and the excitation laser 2 for obtaining a wavelengthin the yellow range by sum frequency generation;

FIG. 10 is a block diagram of a laser light source in the yellow rangeaccording to Embodiment 2-1 of this invention;

FIG. 11 is a block diagram of a laser light source in the yellow rangeaccording to Embodiment 2-2 of this invention;

FIG. 12 is a block diagram of a laser light source in the yellow rangeaccording to Embodiment 2-4 of this invention;

FIG. 13 is a block diagram of a laser light source in the yellow rangeaccording to Embodiment 2-5 of this invention;

FIG. 14 is a diagram showing a 3-dB range with assumed values of theperiod Λ for the wavelength λ₃ as a parameter;

FIG. 15 is a diagram showing normalized conversion efficiency η/η₀ as afunction of the wavelength λ₂ when the period and the wavelength are setas Λ=27 μm and λ₁=1.064 μm, respectively;

FIG. 16 is a block diagram showing a laser light source for generatingmid-infrared light according to one embodiment of this invention;

FIG. 17 is a diagram showing the 3-dB range in Embodiment 3-1;

FIG. 18 is a view showing the polarization dependency of mid-infraredlight outputted in Embodiment 3-1;

FIG. 19 is a block diagram showing an optical absorption analyzeraccording to one embodiment of this invention;

FIG. 20 is a view showing a measurement system of a two-wavelengthdifferential absorption LIDAR according to Embodiment 3-7;

FIG. 21 is a view showing a measurement system of a pesticide residuemeasuring instrument according to Embodiment 3-8;

FIG. 22 is a block diagram showing a laser light source for generating awavelength equal to an oxygen absorption line according to oneembodiment of this invention;

FIG. 23 is a block diagram showing a laser light source equipped with alens and a filter for output;

FIG. 24 is a block diagram showing a laser light source equipped with anoptical fiber for output;

FIG. 25 is a block diagram showing a laser light source according to anembodiment 4-1;

FIG. 26 is a block diagram showing a laser light source according to anembodiment 4-2; and

FIG. 27 is a view showing a method for manufacturing a single-moderidge-type waveguide.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereafter, embodiments of this invention will be described in detailreferring to drawings. In these embodiments, an efficient nonlinearoptical crystal and high-power semiconductor lasers for opticalcommunication are combined. FIG. 3 shows a laser light source accordingto one embodiment of this invention. A laser light source 120 consistsof two excitation lasers 121,122 for exciting a nonlinear opticalcrystal, and a nonlinear optical crystal 123 for generating sumfrequency light or difference frequency light. Incidentally, secondovertone generation of an output beam from one excitation laser inputtedinto a nonlinear optical crystal may be used in some wavelengths.

First Embodiment

In sum frequency generation using a nonlinear crystal, the wavelength λ₃of the sum frequency light is expressed by the following formula, usingthe wavelengths of the two excitation beams represented by λ₁ and λ₂.1/λ₃=1/λ₁+1/λ₂   (1)In order to generate the sum frequency light equivalent to the sodium D1line and D2 line, it is necessary to select λ₁ and λ₂ that giveλ₃=589.592 nm or 588.995 nm in the formula (1) and then combine theexcitation lasers 121,122 of the two wavelengths with the nonlinearoptical crystal 123.

Moreover, to increase the generation efficiency of the sum frequencylight, the following formula must be satisfied among propagationconstants k_(i)=2πn_(i)/λ_(i) (i=1, 2, 3) of the two incident beams inthe nonlinear crystal (λ₁, λ₂), and of the sum frequency light (λ_(b 3))k ₃ =k ₁ +k ₂,   (2)where n_(i) is a refractive index of the nonlinear crystal at λ_(i).However, since the optical medium has a dispersion characteristic, theformula (2) is satisfied only under specific conditions. To be concrete,there is a method in which a polarization direction of any one of theincident beams and the sum frequency light is changed and both therefractive index of ordinary ray and the refractive index ofextraordinary ray are used (for example, see Non-patent document 9).Alternately, a method in which a periodically poled structure is formedin a nonlinear optical crystal, and enhancement of the conversionefficiency is achieved by quasi-phase matching is being used (see Patentdocument 2 and corresponding Patent document 3).

Since the generation intensity of the sum frequency light isproportional to a product of intensities of the two excitation beams,the selection of the two excitation beams is done in such a way that acombination of wavelengths satisfies the formula (1) and the lasers havemuch higher intensities. Among the wavelength bands of existingsemiconductor lasers (for example, they are summarized in Non-patentdocument 10), the wavelength bands in which high power has been madeavailable are (1) 940-nm band, (2) 980-nm band, (3) 1060-nm band, and(5) 1480-nm band. In addition, semiconductor lasers of a 100-mW classare being developed also in (4) 1300-nm band and (6) 1550-nm band.Especially, in the ranges of (4), (5), and (6), DFB (DistributedFeedBack) lasers are being developed, and single longitudinal modeoscillation and wavelength stabilization have been realized. Althougheven in the 800-880 nm range, high-power semiconductor lasers have beendeveloped, if a semiconductor laser in this range is used as anexcitation laser 1, the wavelength of an excitation laser 2 will be setto 1780 nm or more. Since it is difficult to realize high-power andhigh-reliability semiconductor lasers in such a long wavelength band,this combination is excluded.

FIG. 4 shows a relationship of wavelength between the excitation laser 1and the excitation laser 2 for obtaining a wavelength of the sodium Dline wavelength by sum frequency generation. The figure show arelationship for obtaining the sum frequency light, using thewavelengths of the excitation lasers 1 and 2 represented by λ₁ and λ₂,respectively. Wavelength bands of the excitation laser 1 for the above(1) through (6) are designated by 1-(1), 1-(2), 1-(3), 1-(4), 1-(5),and1-(6), respectively, and shown by hatching. In addition, wavelengthbands of the excitation laser 2 of the above (1) through (6) aredesignated as 2-(1), 2-(2), 2-(3), 2-(4), 2-(5), and 2-(6),respectively, and shown by hatching. FIG. 4 indicates that, when using acombination of the excitation laser 1 and the excitation laser 2 suchthat any one of 1-(1) through 1-(6) and any one of 2-(1) through 2-(6)intersect on a curve 30, high-efficiency sum frequency generationbecomes possible.

The ranges of (1) through (6) are set as follows.

(1) 940±10 nm

(2) 980±10 nm

(3) 106±10 nm

(4) 1280 nm to 1350 nm

(5) 1480±10 nm

(6) 1530 nm to 1600 nm

Incidentally, (5) is the O band and (6) is the C band in opticalcommunications. These two wavelength bands are ranges that are beingused most frequently and in which optical parts, such as high-power andhigh-reliability semiconductor lasers, are easily obtained.

For the combination of any one of 1-(1) through 1-(6) and any one of2-(1) through 2-(6) that intersect on the curve 30, it is necessary toconsider that the same sum frequency wavelength can be obtained if thewavelengths of the excitation laser 1 and the excitation laser 2 areexchanged. This consideration leads to a conclusion that combinations of(1) and (6), (2) and (5), and (3) and (4) make intersection on the curve30 and the use of one of these combinations enables the wavelength ofthe sodium D line to be generated efficiently.

Generally, in terms of modes of the laser, there are a single-modeoscillation and a multimode oscillation. The characteristics of the sumfrequency generation light are determined by the characteristics of thetwo excitation semiconductor lasers. In order to perform a single modeoscillation of the sum frequency generation light, the two semiconductorlasers for excitation need to be oscillated in a single mode. For thispurpose, the use of a semiconductor laser having a DFB structure or alaser that uses a fiber Bragg grating in its resonator structure becomesnecessary. On the other hand, in the case of the multimode oscillation,it can be achieved by using a Fabry-Perot type semiconductor laser or asemiconductor laser such that a fiber grating having a reflectionspectrum of a full width of half maximum of about 0.1-0.5 nm is appliedto its resonator structure.

For a nonlinear optical crystal, any crystal that has a large nonlinearoptical constant and is transparent at the wavelengths of the two lasersused for excitation and at the sodium D line wavelength can be used. Asa concrete example, lithium niobium oxide (LiNbO₃, LN), lithium tantalumoxide (LiTaO₃, LT), etc. can be enumerated. Moreover, in order thatthese nonlinear optical crystals may perform sum frequency generationefficiently, it is preferable to have the periodically poled structureand a waveguide structure.

The periodically poled structure is a grating structure in which adirection of polarization is reversed by 180 degrees with a period Λ toa propagation direction of light. With this structure, a quasi-phasematching condition such that the amount of phase mismatching becomeszero is satisfied. Representing refractive indices of a nonlinearoptical crystal at wavelengths λ₁, λ₂, and λ₃ as n₁, n₂, and n₃,respectively, if the structure is made to be the periodically poledstructure that satisfies2πn₃/λ₃=2πn₁/λ₁+2πn₂/λ₂+2πn₂/Λ,   (3)The generation efficiency of the sum frequency light can be maximized.

In addition, since the formation of a waveguide in a nonlinear opticalcrystal enables the incident beams from the excitation lasers to beconfined efficiently, the sum frequency light can be generatedefficiently. The periodically poled structure can be realized by anelectric field application method, and the waveguide structure can berealized by the proton exchange method, a dry etching method, amachining method using a dicing saw, or the like. The method formanufacturing a waveguide will be described later as a fifth embodiment.

Moreover, the generation of the sum frequency light needs coupling ofthe two semiconductor laser beams and coupling of them to an LNwaveguide. These techniques have been established as opticalcommunication device technologies, featuring that there is not a largeobstacle in implementing the coupling.

For example, the linewidth of the existing semiconductor DFB laser is 1MHz, and the linewidth of the external mirror resonator-typesemiconductor laser using a fiber Bragg grating is about 100 kHz. Thelinewidth of the sum frequency light when these lasers are used asexcitation lasers is a few MHz or less, which is estimated byconvolution integral of the two linewidths of the excitation beams. Inthe case where the refractive index at the sodium D line (wavelength:589.3 nm, frequency: approximately 500 THz) by interferometry, itsmeasurement accuracy is given by a ratio of the linewidth to thefrequency of the laser beam used. Assuming that the linewidth is 5 MHz,the measurement accuracy is 10⁻⁸. Therefore, according to thisembodiment, it becomes possible to improve the accuracy of therefractive index measurement by about two orders of magnitude comparedto the present state.

As explained in the foregoing, the coherent beams having wavelengths ofthe sodium D1 line and D2 line can be generated efficiently in a highlystable manner by selection of existing laser devices together withimprovement of characteristics of a nonlinear optical crystal, whichmakes it possible to minimize a laser light source and to enhance theaccuracy of the refractive index measurement.

Embodiment 1-1

FIG. 5 shows a laser light source of the sodium D-line wavelengthaccording to Embodiment 1-1 of this invention. The laser light source isconstructed with two excitation lasers 140,141, an LN144 whosepolarization was reversed periodically, lenses 142 a, 142 b forcollimating laser beams of the excitation lasers 140, 141, a multiplexer143 for multiplexing two laser beams, and a filter 145 for separatingthe laser beams of the excitation lasers 140, 141 that passed throughthe LN144 and the sum frequency light generated in the LN 144.

The wavelength λ₁ of the excitation laser 140 and the wavelength λ₂ ofthe excitation laser 141 are specified of to be of a combination thatsatisfies1/λ₁+1/λ₂=1/(589.3±2.0).Moreover, λ₁ and λ₂ are in wavelength bands that satisfy any one of thefollowing sets.

λ₁=976±10 nm, λ₂=1485±20 nm

λ₁=1064±10 nm, λ₂=1320±20 nm

λ₁=940±10 nm, λ₂=1565±35 nm

The semiconductor laser of λ₂ may be a DFB laser.

When the excitation laser 140 is set so that a wavelength λ₁=1064 nm andthe incident intensity on the LN 144 is 50 mW and the excitation laser141 is set so that a wavelength λ₂=1320 nm and the incident intensity onthe LN 144 is 70 mW, the sum frequency light whose wavelength λ₃ was589.1 nm and output was 20 μW was obtained.

Embodiment 1-2

FIG. 6 shows a laser light source of the sodium D-line wavelengthaccording to Embodiment 1-2 of this invention. A difference from thelaser light source of Embodiment 1-l lies in a nonlinear opticalcrystal. For the nonlinear optical crystal, a periodically poled LNwaveguide 151 such that a waveguide was formed in an LN crystal wasused. Moreover, the nonlinear optical crystal has a lens 150 thatcouples the incident laser beam to the periodically poled LN waveguide151 efficiently and a lens 152 that collimates the emitted beam from theperiodically poled LN waveguide 151.

When the excitation laser 140 was set so that a wavelength λ₁=1064 nmand the incident intensity on the LN 144 was 50 mW and the excitationlaser 141 was set so that a wavelength λ₂=1320 nm and the incidentintensity on the LN 144 was 70 mW, the sum frequency light whosewavelength λ₃ was 589.1 nm and output was 10 mW was obtained.

Embodiment 1-3

To construct Embodiment 1-3, in the configurations of Embodiment 1-1 andEmbodiment 1-2 (FIG. 4 and FIG. 5), the excitation laser 140 isspecified to be a laser using a Nd ion whose wavelength is near 1064 nm(for example, Nd—YAG laser) and the excitation laser 141 is specified tobe a semiconductor laser whose wavelength is 1300±10 nm.

Embodiment 1-4

FIG. 7 shows a laser light source of the sodium D-line wavelengthaccording to Embodiment 1-4 of this invention. To construct Embodiment1-4, polarization maintaining fibers (or single mode fibers) 161,163 anda multiplexer 162 are used in order to couple the two laser beams to theperiodically poled LN waveguide 151 in the configuration of Embodiment1-2. The beam emitted from the polarization maintaining fiber 163 isincident directly on a facet of the periodically poled LN waveguide 151,or is coupled thereto with a lens 164.

Embodiment 1-5

FIG. 8 shows a laser light source of the sodium D-line wavelengthaccording to Embodiment 1-5 of this invention. Embodiment 1-5 is anexample of further application of Embodiment 1-4. In excitation lasers170,171, AR coatings of a reflectance of 2% or less are applied onlight-emitting side facets 170 a, 171 a and HR coatings of a reflectanceof 90% or more are applied on opposite facets 170 b, 171 b. An output ofthe excitation laser 170 (171) is coupled, through a lens 172 a (172 b),to a polarization maintaining fiber (or single mode fiber) 173 (174)such that at its facet or at some midpoint therein a fiber Bragg gratingwas formed. Thus, resonators are constructed between the HR coating onthe facet 170 b (171 b) and the fiber Bragg grating.

An oscillation wavelength of each laser is controlled by the reflectionspectrum of the fiber Bragg grating. At this time, the centralwavelengths of the reflection spectra of the fiber Bragg gratings arespecified to be any one of the following pairs.

976±10 nm, 1485±20 nm

1064±10 nm, 1320±20 nm

940±10 nm, 1565±35 nm

The linewidths (full widths at half maximums) of the reflection spectraare specified to be 0.3 nm or less, respectively.

Second Embodiment

The configuration of a laser light source in the yellow range accordingto one embodiment of this invention is as shown in FIG. 3. In order togenerate the sum frequency light that corresponds to the yellow range,it is necessary to select the wavelengths λ₁, λ₂ that will generate λ₃of 546.1 nm, 560.0 nm, or 585.0 nm in the formula (1) and combine thetwo excitation lasers 121, 122 of two wavelengths and the nonlinearoptical crystal 123.

FIG. 9 shows a relationship of wavelength between the excitation laser 1and the excitation laser 2 to obtain a wavelength in the yellow range bysum frequency generation. Representing the wavelength of the excitationlaser 1 and the wavelength of the excitation laser 2 by λ₁ and λ₂,respectively, a relationship for obtaining the sum frequency light isindicated by the curve 30. Moreover, wavelength bands of the excitationlaser 1 of the above (1) through (6) are designated as 1-(1), 1-(2),1-(3), 1-(4), 1-(5), and 1-(6), respectively, and shown by hatching. Inaddition, wavelength bands of the excitation laser 2 of the above (1)through (6) are designated as 2-(1), 2-(2), 2-(3), 2-(4), 2-(5), and2-(6), respectively, and shown by hatching. Incidentally, the ranges of(1) though (6) are the same as those in FIG. 4.

FIG. 9 indicates that high-efficiency sum frequency generation becomespossible by using a combination of the excitation laser 1 and theexcitation laser 2 such that anyone of 1-(1) through 1-(6) and any oneof 2-(1) through 2-(6) intersect on a curve 21 giving λ₃=546.1 nm, or ona curve 22 giving λ₃=560.0 nm, or on a curve 23 giving λ₃=585.0 nm.

For the combination of any one of 1-(1) through 1-(6) and any one of2-(1) through 2-(6) that intersect on the curves 21-23, it is necessaryto consider that the same sum frequency wavelength can be obtained ifthe wavelengths of the excitation laser 1 and the excitation laser 2 areexchanged. This consideration leads to a conclusion that when any one ofcombinations of (1) and (4), (2) and (4), (3) and (4), and (1) and (6)is used, a wavelength in the yellow range can be generated efficiently.

As described in the foregoing, the selection of existing laser devices,together with improvement in characteristics of a nonlinear opticalcrystal, enables a coherent beam in the yellow range to be generatedefficiently in a highly stable manner, which makes it possible tominiaturize a laser light source and improve accuracy of refractiveindex measurement.

Embodiment 2-1

FIG. 10 shows a laser light source in the yellow range according toEmbodiment 2-1 of this invention. The laser light source is constructedwith two excitation lasers 240, 241, an LN 244 whose polarization isreversed periodically, lenses 242 a, 242 b each for collimating one ofthe laser beams of the excitation lasers 240, 241, a multiplexer 243 formultiplexing two laser beams, and a filter 245 for separating the sumfrequency light generated in the LN 244 from the laser beams of theexcitation lasers 240, 241 that passed through the LN 244.

The wavelength λ₁ of the excitation laser 240 and the wavelength λ₂ ofthe excitation laser 241 are specified to be a pair that satisfies1/λ₁+1/λ₂=1/(546.1±5.0)Moreover, the pair of λ₁ and λ₂ is specified to be any one of theaforesaid combinations of (1) through (4) and be in the range thatsatisfiesλ₁=940±10 nm, λ₂=1320±20 nmThe semiconductor laser of λ₂ may be a DFB laser.

When the excitation laser 240 was set so that a wavelength λ₁=940 nm andthe incident intensity on the LN 244 was 40 mW and the excitation laser241 was set so that a wavelength λ₂=1320 nm and the incident intensityon the LN 244 was 70 mW, the sum frequency light whose wavelength λ₃ was546.1 nm and output was 20 μW was obtained.

Embodiment 2-2

FIG. 11 shows a laser light source in the yellow range according toEmbodiment 2-2 of this invention. A difference from the laser lightsource of Embodiment 2-1 lies in a nonlinear optical crystal. Regardingthe nonlinear optical crystal, a periodically poled LN waveguide 251such that a waveguide is formed in an LN crystal is used. Moreover, thelaser light source has a lens 250 for efficiently coupling the incidentbeams to the periodically poled LN waveguide 251 and a lens 252 forcollimating the emitted beam from the periodically poled LN waveguide251.

When the excitation laser 240 was set so that a wavelength λ₁=940 nm andthe incident intensity on the LN 244 was 40 mW and the excitation laser241 was set so that a wavelength λ₂=1320 nm and the incident intensityon the LN 244 was 70 mW, the sum frequency light whose wavelength λ₃ was546.1 nm and output was 10 mW was obtained.

Embodiment 2-3

To construct Embodiment 2-3, the excitation laser 240 is specified to bea laser using a Nd ion whose wavelength is near 1064 nm (for example,Nd—YAG laser) and the excitation laser 241 is specified to be asemiconductor laser whose wavelength is 1320±20 nm in the configurationsof Embodiment 2-1 and Embodiment 2-2 (FIG. 10 and FIG. 11). Therefore,this embodiment uses the aforesaid combination of (3) and (4), and thesum frequency light of a wavelength λ₃=585.0 nm in the yellow range canbe obtained.

Embodiment 2-4

FIG. 12 shows a laser light source in the yellow range according toEmbodiment 2-4 of this invention. In the configuration of Embodiment2-2, polarization maintaining fibers (or single mode fibers) 261,263 anda multiplexer 262 were used in order to couple two laser beams to theperiodically poled LN waveguide 251. The beam emitted from thepolarization maintaining fiber 263 is incident directly on the facet ofthe periodically poled LN waveguide 251, or is coupled thereto with alens 264.

Embodiment 2-5

FIG. 13 shows a laser light source in the yellow range according toEmbodiment 2-5 of this invention. This is an example of furtherapplication of Embodiment 2-4. In excitation lasers 270, 271, ARcoatings of a reflectance of 2% or less are applied on light-emittingside facets 270 a, 271 a and HR coatings of a reflectance of 90% or moreis applied on opposite facets 270 b, 271 b. An output of the excitationlaser 270 (271) is coupled, through a lens 272 a (272 b), to apolarization maintaining fiber (or single mode fiber) 273 (274) suchthat at its facet or at some midpoint therein a fiber Bragg grating wasformed. Thus, resonators are constructed between the HR coating on thefacet 270 b (271 b).

An oscillation wavelength of each laser is controlled by means of areflection spectrum of the fiber Bragg grating. At this time, thecentral wavelengths of the reflection spectra of the fiber Bragggratings are specified to be any one set of the following pairs.

940±10 nm, 1320±20 nm

980±10 nm, 1320±20 nm

1064±10 nm, 1320±20 nm

940±10 nm, 1550±30 nm

The linewidths (full widths at half maximum) are specified to be 0.3 nmor less.

Third Embodiment

In a method for generating mid-infrared light by difference frequencygeneration using a nonlinear optical crystal and two excitation laserbeams, a relationship among the wavelengths λ₁, λ₂ of two excitationlaser beams and the wavelength λ₃ of the generated mid-infrared light isgiven by the following formula. $\begin{matrix}\left\lbrack {{Formula}\quad 1} \right\rbrack & \quad \\{\frac{1}{\lambda_{3}} = {\frac{1}{\lambda_{1}} - \frac{1}{\lambda_{2}}}} & (3)\end{matrix}$Here, the wavelength λ₁ may be larger or smaller than the wavelength λ₂.However, in order to satisfy λ₃>0 for convenience' sake, it is assumedthat the wavelengths λ₁ and λ₂ satisfy: λ₁<λ₂. In order to generate thedifference frequency light λ₃ efficiently, the light needs to satisfythe following phase matching condition.

[Formula ]k ₃ =k ₁ −k ₂   (4)In the formula (4), ki (i=1, 2, and 3) is a propagation constant of eachlaser beam propagating in the nonlinear crystal and satisfies thefollowing formula with the refractive index of the nonlinear opticalcrystal at k_(i) represented by n₁ . $\begin{matrix}\left\lbrack {{Formula}\quad 3} \right\rbrack & \quad \\{k_{i} = {\frac{2\pi}{\lambda_{i}}n_{i}}} & (5)\end{matrix}$However, it is generally difficult to satisfy the formula (4) due to adispersion characteristic that a crystal possesses.

As a method for solving this, a quasi-phase matching method in which anonlinear crystal is polarized and inversely polarized periodically isbeing used. For the quasi-phase matching method, ferroelectric crystals,such as LiNbO₃, are advantageous. Polarities of nonlinear opticalconstants of these crystals correspond to polarities of spontaneouspolarization. When this spontaneous polarization is modulated with aperiod Λ in the propagation direction of light, the phase matchingcondition is expressed by the following formula. $\begin{matrix}\left\lbrack {{Formula}\quad 4} \right\rbrack & \quad \\{k_{3} = {k_{1} - k_{2} - \frac{2\pi}{\Lambda}}} & (6)\end{matrix}$When specific wavelengths λ₁, λ₂ are used as excitation beams, theformulas (3) and (6) can be satisfied simultaneously and hence thedifference frequency light λ₃ can be generated efficiently.

However, when the wavelengths λ₁, λ₂ are varied to obtain a differentwavelength λ₃ of the difference frequency light, if there is fluctuationin the wavelengths λ₁, λ₂, the three wavelengths no longer satisfy theformula (6) and the intensity of the difference frequency light λ₃reduces. Here, a relationship among the wavelengths λ₁, λ₂, and λ₃, theperiod Λ, and the generation efficiency η of the difference frequencylight is considered. First, the amount of phase mismatching Λk isdefined as follows. $\begin{matrix}\left\lbrack {{Formula}\quad 5} \right\rbrack & \quad \\{{\Delta\quad k} = {k_{3} - k_{1} + k_{2} + \frac{2\pi}{\Lambda}}} & (7)\end{matrix}$At this time, representing a sample length as 1, the generationefficiency η of the difference frequency light depends on a product ofΛk and 1 and is expressed by the following formula. $\begin{matrix}{\left\lbrack {{Formula}\quad 6} \right\rbrack{\eta = {\eta_{0}\frac{\sin^{2}\left( \frac{\Delta\quad{kl}}{2} \right)}{\left( \frac{\Delta\quad{kl}}{2} \right)^{2}}}}} & (8)\end{matrix}$In the formula (8), η₀ is a generation efficiency of the differencefrequency light when Λk=0, and is determined by the nonlinear opticalconstant of a crystal, such as LiNbO₃, the excitation beam intensity,the sample length, etc. Therefore, in the same sample, since the periodΛ is fixed, any change in either the wavelength λ₁ or the wavelength λ₂increases or decreases Λk, bringing reduction in the generationefficiency η. Ranges of the wavelengths λ₁, λ₂ that give η=0.5η₀ for agiven period Λ, that is, ranges that satisfy $\begin{matrix}\left\lbrack {{Formula}\quad 7} \right\rbrack & \quad \\{\frac{\sin^{2}\left( \frac{\Delta\quad{kl}}{2} \right)}{\left( \frac{\Delta\quad{kl}}{2} \right)^{2}} \geq 0.5} & (9)\end{matrix}$are called 3-dB ranges for the period Λ. If this 3-dB range can bewidened, the wavelength of the difference frequency light λ₃ can be madevariable without reducing the generation efficiency.

In the following discussion, a case where z-cut LiNbO₃ is used and thepolarization directions of the two excitation beams and the differencefrequency light all lie in the direction of c-axis of the crystal willbe treated. At this time, the propagation characteristics of the twoexcitation beams and the difference frequency light are determined bythe extraordinary ray refractive index n_(e). n_(e) is given by theSellmeier's equation $\begin{matrix}\left\lbrack {{Formula}\quad 8} \right\rbrack & \quad \\{{n_{e}^{2}(\lambda)} = {4.5567 - {2.605 \times 10^{- 7}T^{2}} + \frac{0.097 + {2.7 \times 10^{- 8}T^{2}}}{\lambda^{2} - \left( {0.201 + {5.4 \times 10^{- 8}T^{2}}} \right)^{2}} - {2.24 \times 10^{- 2}\lambda^{2}}}} & (10)\end{matrix}$Here, T denotes temperature (K) and the wavelength λ₃ is expressed inμm.

FIG. 14 shows the 3-dB range obtained assuming the period Λ as severalvalues with the wavelength λ₃ as a parameter. The 3-dB ranges of thewavelengths λ₁, λ₂ are given from the formulas (1), (5), and (7). Thefigure shows a relationship between the wavelengths λ₁, λ₂ by dottedlines that give the wavelengths λ₃ of the difference frequency light of2.0 μm, 2.5 μm, 3.0 μm, 3.5 μm, 4.0 μm, 4.5 μm, 5.0 μm, 5.5 μm, and 6.0μm calculated from the formula (3) at room temperature. Moreover, the3-dB ranges for periods Λ=26 μm, 27 μm, 28 μm, 29 μm, and 30 μm arecalculated by the formulas (7) and (9), and these ranges are shown byhatching. Device length was set to 10 mm.

A conversion efficiency of η=η₀ when the phase matching condition iscompletely satisfied exists in an almost middle part of the 3-dB range.That is, in difference frequency generation in LiNbO₃ that has theperiodically poled structure of a period Λ, the quasi-phase matchingelement of a period Λ is used. In the case where a desired differencefrequency light λ₃is obtained, the wavelengths λ₁, λ₂ to achieve η=0.5η₀are obtained from the formulas (3), (7), and (9), and the 3-dB range forthe period Λ is given by intersections of the curves of the formula (3)that gives the desired difference frequency light λ₃.

As an example, consider a case where the difference frequency light of awavelength λ₃=3.0 μm is generated using LiNbO₃ having the periodicallypoled structure of a period Λ=28 μm. A range of the wavelengths λ₁, λ₂where a dotted line of a wavelength λ₃=3.0 μm and the 3-dB range of aperiod Λ=28 μm intersect (apart that is circled and designated by asymbol A) give η=0.5η₀.

Next, concrete conditions will be shown. The generation intensity indifference frequency generation is proportional to a product of twoexcitation beam intensities. Because of this, the Nd—YAG laser(wavelength of 1.064 μm) that can easily achieve high intensity wasmainly used in hitherto reported examples. Here, the case where thewavelength λ₁ is fixed as λ₁=1.064 tm and the wavelength λ₂ is varied toachieve tunable difference frequency light λ₃is considered. When LiNbO₃having the periodically poled structure of a period Λ is used, η=0.5η₀is achieved at the wavelength λ₂ in a range where the 3-dB range of aperiod Λ shown by hatching in FIG. 14 and a straight line B of awavelength λ₁=1.064 μm intersect.

FIG. 15 shows normalized conversion efficiency η/η₀ as a function of thewavelength λ₂ when a period Λ=27 μm and a wavelength λ₁=1.064 μm areset, respectively. The width of the wavelength λ₂ that satisfiesη=0.51η₀ is only about 2 nm, and consequently the amount of tunabilityof the difference frequency light λ₃ is limited to about 20 nm. Inaddition, when the period Λ is changed to any of 28 μm, 29 μm, and 30μm, as long as a wavelength λ₁=1.064 μm is assumed, the width of awavelength λ₂ that satisfies η=0.5η₀ is only about 2 nm in any case;therefore, the amount of tunability of the difference frequency light λ₃is limited similarly.

However, examination of FIG. 14 indicates that there is a range where atunable range of the difference frequency light λ₃ can be widenedconsiderably if the wavelength λ₁ is fixed and the wavelength λ₂ isvaried. That is, if the straight line indicating a constant wavelengthλ₁ and the 3-dB range of a period Λ intersect in a wider range, thetunable range width of the difference frequency light λ₃ will increasedramatically. The 3-dB range of a period Λ=25.5-29 μm is almost parallelto the vertical axis at a wavelengths λ₁ of 0.9-1.0 μm, and accordinglyintersects widely the straight line indicating a constant wavelength λ₁in this wavelength band of 0.9-1.0 μm. That is, even if using theperiodically poled structure LiNbO₃ having a single period Λ, when thewavelength λ₁ is fixed in the range of 0.9-1.0 μm and the wavelength λ₂is varied in the range of 1.3-1.8 μm, the difference frequency light λ₃can be tuned efficiently satisfying the phase matching condition inalmost whole range of 1.3 μm <λ₂<1.8 μm.

For example, when a period Λ=27 μm and a wavelength λ₁=0.94 μm are set,the normalized conversion efficiency for a wavelength λ₂ becomes η=0.5η₀in the wavelength band of λ₂>1.43 μm and the difference frequency lightcan be generated in a wide wavelength band of almost 2-3 μm. Inaddition, near the wavelength λ₃=3 μm, it becomes possible to generateit with a single period Λ by temperature adjustment, as will bedescribed later.

As explained above, the laser light source equipped with the firstlaser, the second laser, and the nonlinear optical crystal having theperiodically poled structure of a single period can tune the laser beamin the mid-infrared region so as to be in the wavelength band of 2-3 μmby changing the wavelength of one of the lasers.

Embodiment 3-1

FIG. 16 shows a laser light source for generating mid-infrared lightaccording to one embodiment of this invention. The laser light sourcecomprises: a semiconductor laser 310 of a wavelength λ₁ (λ₁ is specifiedto be in a 0.94 μm wavelength band); a semiconductor laser 311 of awavelength λ₂ (λ₂ is specified to be in 1.45-1.60 μm wavelength band andtunable); a multiplexer 318 for multiplexing the output beams of thesemiconductor lasers 310,311; and a LiNbO₃ bulk crystal 321 with theperiodically poled structure of a single period that allows themultiplexed beams to enter thereinto and generates the differencefrequency light, i.e., mid-infrared light. The output of thesemiconductor laser 310 is connected to the multiplexer 318 through acoupling lens system 312,313 and a polarization maintaining fiber 316.The output of the semiconductor laser 311 is connected to themultiplexer 318 through a coupling lens system 314,315 and apolarization maintaining fiber 317.

In the semiconductor laser 310, a high reflective film of a reflectanceof 90% or more is formed on its facet 310A, and a low reflective film ofa reflectance of 2% or less is formed on its opposite facet 310B. Thepolarization maintaining fiber 316 is provided with a fiber Bragggrating 316A, so that the wavelength stability is improved. It isfurther possible to connect a fiber amplifier in the middle of thepolarization maintaining fiber 317, as needed, to boost the output lightof the semiconductor laser 311.

The output of the multiplexer 318 is connected to the LiNbO₃ bulkcrystal 321 through an optical fiber 319 and a coupling lens system 320.Incidentally, the output of the LiNbO₃ bulk crystal 321 is connected toa spectrometer 325 through a coupling lens system 322,324 and an opticalfiber 323 in order to measure the output beam that is mid-infraredlight.

As shown by the straight line C in FIG. 14, when a wavelength λ₁ isspecified to be in the 0. 94-μm wavelength band, and when a period Λ ofthe LiNbO₃ bulk crystal 321 is 27 μm, the aforesaid 3-dB range can beobtained with a single period Λ even if the wavelength of thesemiconductor laser 311 is varied in the range of 1.45-1.60 μm. In otherwords, mid-infrared light can be obtained in the wide wavelength bandwith a single period Λ. The figure shows that with a wavelength λ₁ inthe 0.94-μm wavelength band, when the wavelength λ₂ is varied in therange of 1.45 μm to 1.60 μm, the wavelength λ₃ of generated mid-infraredlight will cover a wide range of 2.3-2.7 μm.

FIG. 17 shows the 3-dB range in Embodiment 1. The vertical axisindicates mid-infrared light intensity, and the horizontal axisindicates the wavelength λ₂ of the semiconductor laser 311. As can beexpected from calculation results in FIG. 14, with the LiNbO₃ bulkcrystal 321 having a single period Λ, mid-infrared light with almostconstant intensity can be obtained in a wide wavelength band of 1.45μm<λ₂<1.60 μm. The output of the semiconductor laser 311 is constant inthe whole wavelength band. A variation of 1.45 μm<λ₂<1.60 μm correspondsto a variation of 2.7 μM>λ₃ of mid-infrared light>2.3 μm. The wavelengthof generated mid-infrared light is checked with the spectroscope 325. Inthis embodiment, the LiNbO₃ bulk crystal 321 of a device length of 10 mmwas used. The conversion efficiency was 1%/W in the whole wavelengthband.

In the case where a difference-frequency-generation experiment like thisembodiment is conducted, maximum mid-infrared light is generated whenpolarization directions of the two excitation beams coincide with eachother. Here, if the polarization direction of the semiconductor laser310 is inclined by an angle θ while the polarization direction of thesemiconductor laser 311 is fixed, the light intensity I₃ of themid-infrared light will be expressed by the following formula, using thelight intensity of the semiconductor laser 310 represented by I₁ and thelight intensity of the semiconductor laser 311 represented by I₂,

[Formula 9]I ₃ ∝I ₁ I ₂ cos² θ  (11)The formula (11) can be used as means for checking the generation of themid-infrared light. FIG. 18 shows polarization dependency of themid-infrared light outputted in Embodiment 3-1. It was confirmed that anexperimental result was mostly in agreement with what was obtained bycalculation.

Embodiment 3-2

In Embodiment 3-1, the wavelength band of the outputted mid-infraredlight was 2.3-2.7 μm. However, the wavelength band can be expandedfurther by changing the period Λ of the LiNbO₃ crystal. In Embodiment3-2, the period Λ of the LiNbO₃ bulk crystal 321 shown in FIG. 16 wasset to 26 μm. The semiconductor laser 310 was specified to be a devicecapable of tuning its wavelength in a very small range in a 0.91-μmwavelength band, and the semiconductor laser 311 was specified to be adevice capable of tuning its wavelength in a wide range in thewavelength band of 1.30-1.68 μm.

This device can deliver mid-infrared light having almost constantintensity in as wide a 3-dB range as the wavelength band of 1.30μm<λ₂<1.68 μm with the LiNbO₃ bulk crystal 321 that uses only one periodΛ. Since the wavelength λ₂ was varied in the range of 1.30-1.68 μm, thewavelength λ₃ of the mid-infrared light in a range of 3.1-2.0 μm wasable to be obtained. In this embodiment, the LiNbO₃ bulk crystal 321 ofa device length of 10 mm was used. The conversion efficiency was 1%/W inthe whole wavelength band.

Note that the refractive index of LiNbO₃ crystal varies withtemperature, as can be seen from the formula (10), and consequently theeffective period Λ also varies with this. Therefore, if the temperatureof LiNbO₃ crystal is adjusted minutely, the effective one period Λ canbe varied. Therefore, even when the difference frequency generation isperformed in a LiNbO₃ crystal having a single period Λ, high conversionefficiency can be maintained. As shown in FIG. 14, there is a range inwhich the conversion efficiency cannot be maintained high if thewavelength of the semiconductor laser 310 is fixed (for example, a rangein which the characteristic curve is not in parallel completely to thevertical axis as in the case of a period Λ=28.29 μm). In thisconnection, if the temperature of the LiNbO₃ bulk crystal 321 isadjusted so that the effective period Λ to the wavelength of thesemiconductor laser 310 is optimized, the high conversion efficiency canalways be maintained.

In Embodiment 3-2, the period Λ was varied by a step of 0.1 μm between25.5 μm and 29.3 μm under a suitable temperature adjustment and a beamof the difference frequency is generated using the LiNbO₃ bulk crystal321 having a period Λ. As a result, when a wavelength λ₁ is suitablyselected in the range of 0.9 -1.0 μm for each period Λ and thewavelength λ₂ is varied in the range of 1.27-1.80 μm according to this,the wavelength λ₃ of the mid-infrared light can be continuously obtainedin the range of 3.1-2.0 μm. However, in the period Λ exceeding 28.5 μm,a portion in the characteristics curve parallel to the vertical axistends to decrease, as shown in FIG. 14. Therefore, contribution of thetemperature control necessary to obtain a beam of the differencefrequency of a constant intensity becomes large gradually with theperiod Λ. A temperature change of 100° C. corresponded to a change asmuch as 0.005 μm of the wavelength λ₁.

Embodiment 3-3

If a wavelength converter element is changed to a waveguide type fromLiNbO₃ crystal of the bulk type and the wavelength converter isconfigured in the same manner as Embodiments 3-1 and 3-2, mid-infraredlight can be obtained more efficiently. Embodiment 3-3 used an opticalsystem such that the LiNbO₃ bulk crystal 321 shown in FIG. 16 waschanged to a waveguide element. The device length of the LiNbO₃waveguide was set to 10 mm, a cross-sectional size of the core was setto 8 μm×8 μm, and the period Λ was set to 26 μm. The semiconductor laser310 was specified to be tunable in a very small range in the 0.91-μmwavelength band, and the semiconductor laser 311 was specified to betunable in a wide range in the wavelength band of 1.3-1.65 μm.

Regarding the 3-dB range in the waveguide element, mid-infrared light 3having an almost constant intensity was obtained in the wavelength bandof 3.1-2.0 μm for λ₁ in the 0.91-μm wavelength band and for λ₂ in a widewavelength band of 1.3 μm <λ₂<1.65 μm under an appropriate temperatureadjustment. The conversion efficiency was improved in the wholewavelength band, showing improvement by two orders of magnitude comparedto the bulk element.

Moreover, the period Λ was changed in a range of 25.5-29.3 μm by a stepof 0.1 μm, and the mid-infrared light was generated under suitabletemperature adjustment using a LiNbO₃ waveguide having a period Λ. Theresult shows that the wavelength λ₃ of the mid-infrared light can becontinuously obtained in the range of 3.1-2.0 μm when the wavelength λ₁is suitably selected from a range of 0.9-1.0 μm for each period Λ andthe wavelength λ₂ is varied in the range of 1.27-1.80 μm according tothe wavelength λ₁.

Embodiment 3-4

As shown in FIG. 14, the phase matching curve has an area in which thecurve makes an abrupt bend. The use of this area does not give a largemerit particularly in terms of wavelength tunability. However, inperforming the difference frequency generation, the tolerance ofwavelength stability in each of the two excitation beams is improvedlargely, especially bringing an effect for improvement in the toleranceof a short wavelength-side semiconductor laser. For example, in FIG. 14,in the case of a period Λ=27 μm, for the wavelength λ₂ of thesemiconductor laser 311 in the range of 1.45-1.8 μm, when the wavelength2 varies, the wavelength λ₂ will not go out of the 3-dB range. On theother hand, when the wavelength λ₁ of the semiconductor 310 varies evenslightly, it will cause the wavelength λ₃ to go out of the 3-dB range.However, when the wavelength λ₂ is in a curved part near 1.35 μm, therearises an advantage that the tolerance in the wavelength variation forthe 3-dB range is doubled also for the wavelength λ₁ that is a half ofthe wavelength λ₂. At the same time, the amount of temperatureadjustment of the LiNbO₃bulk crystal 321 also decreases. Here, note thatthe tolerance for the wavelength λ₂ decreases in this case, but stillkeeps a sufficient width from the point of view of the stability of thecommercially available normal laser light source.

Embodiment 3-4 uses an optical system in which the reflective films onthe facets 310A, 310B of the semiconductor laser 310A and the fiberBragg grating 316A of the polarization maintaining fiber 316 are removedfrom the previously described optical system. The fiber Bragg grating isthe device with which a beam of the designed wavelength can be obtainedselectively, and was used to suppress the variation in the wavelength kin Embodiment 3-1. Therefore, when the fiber Bragg grating 316A isremoved, there might be a case where a stable 3-dB range is hard toobtain. However, in Embodiment 3-4, the laser light source can performsufficiently stable operation, not getting out of the 3-dB range, evenwithout a configuration for stabilizing the wavelength like this. Inthis embodiment, the period Λ of the LiNbO₃ bulk crystal 321 was set to27 μm, the wavelength of the semiconductor laser 310 was set to 0.945μm, and the wavelength of the semiconductor laser 311 was set to 1.35μm.

Embodiment 3-5

With use of a laser light source for generating mid-infrared lightaccording to this invention, NO_(x) that is an environmental gas can bedetected accurately. Since the fundamental absorption of NO_(x) gasexists in a wavelength of longer than 5 μm, it is convenient to use thefollowing reaction formulas to detect it, considering the absorptionproperty of LiNbO₃ (light of a wavelength of 5.4 μm or more hardly passthrough LiNbO₃).4NO+4NH₃+O₂→4N₂+6H₂O   (12)6NO₂+8NH₃→7N₂+12H₂O   (13)

That is, since NO_(x) is decomposed by NH₃ with a catalyst,concentrations of NO and NO_(x) can be calculated indirectly by checkingthe quantity of consumed NH3 or the quantity of newly generated H₂O.Moreover, a fact that overtones of the fundamental absorption of NO andNO₂ reside in the wavelengths of 2-3 μm can be used to detect them.Then, if there is a laser light source that is tunable in wavelengths of2-3 μm, the aforesaid absorption of the gases can be checked together.The major fundamental absorption wavelength, the wave number, and thename of absorption of gases are as follows.

H₂O: 2.662 μm, 3756 cm⁻¹, anti-symmetry stretching vibration

H₂O: 2.734 μm, 3657 cm⁻¹, totally symmetry stretching vibration

NH₃: 2.904 μm, 3444 cm⁻¹, double degenerated vibration

NH₃: 2.997 μm, 3337 cm⁻¹ , totally symmetry vibration

NO: 5.330 μm, 1876 cm⁻¹, anti-symmetry stretching vibration,overtone=2.665 μm

NO₂: 6.180 μm, 1618 cm⁻¹, anti-symmetry stretching vibration,overtone=3.090 μm

FIG. 19 shows an optical absorption analyzer according to one embodimentof this invention. This figure particularly shows an optical system fordetecting NO, gas concentration. A gas cell 344 in which a measured gasis enclosed has an optical path length of up to 18 m using reflectors onboth ends thereof. A reactive gas is led into the gas cell 344 from agas removal tube 346 and discharged to a gas exhaust tube 348 by a pump347. When the pump is employed, the pressure in the gas cell can bechanged. In the gas removal tube 346, NO_(x) is removed by reactions offormula (12) or (13). A detector 349 is a HgCdTe detector formid-infrared light.

The laser light source comprises: a semiconductor laser 330 of awavelength λ₁ (λ₁ is specified in the 0.94-μm wavelength band, fixed); asemiconductor laser 331 of a wavelength λ₂ (λ₂ is specified in the rangeof 1.28-1.46 μm, tunable), a multiplexer 338 for multiplexing the outputbeams of the semiconductor lasers 330, 331, and a LiNbO₃ bulk crystal341 of a period Λ=26 μm that generates mid-infrared light. The output ofthe semiconductor laser 330 is connected to the multiplexer 338 througha coupling lens system 332, 333 and a polarization maintaining fiber336, and the output of the semiconductor laser 331 is connected to themultiplexer 338 through a coupling lens systems 334, 335 and apolarization maintaining fiber 337.

In the semiconductor laser 330, a high reflective film of a reflectanceof 90% or more is formed on its facet 330A, and a low reflective film ofa reflectance of 2% or less is formed on its opposite facet 330B. Thepolarization maintaining fiber 336 is provided with a fiber Bragggrating 336A, so that the wavelength stability is improved. The outputof the multiplexer 338 is connected to the LiNbO₃ bulk crystal 341through an optical fiber 339 and a coupling lens system 340. The outputof the LiNbO₃ bulk crystal 341 is connected to a gas cell 344 through acoupling lens system 342 and an optical fiber 343.

In the description of Embodiment 3-5, first measurement resultsaccompanying removal of NO₂ gas will be shown. The measurement isperformed after dividing it into the following three stages.

(i) Only NO₂ gas is introduced into the gas removal tube without givinga catalyst and NH₃ gas.

(ii) NO₂ gas is introduced into the gas removal tube without giving acatalyst while NH₃ gas is given.

(iii) NO₂ gas is introduced into the gas removal tube while a catalystand NH₃ gas are given. The stage (iii) corresponds to a state where nochemical reaction occurs when the wavelength of the semiconductor laser331 is adjusted to 1.290 μm, and overtone absorption of theanti-symmetry stretching vibration of NO₂ can be detected at awavelength 3.090 μm. On the other hand, even when the wavelength of thesemiconductor laser 331 is adjusted again to match absorptionwavelengths of NH₃ or H₂O, absorptions of these two gases are notobserved.

In the stage (ii), even when NH₃ is given, no chemical reactionprogresses because there is no catalyst, so absorptions of unreacted NO₂and NH₃ will be observed. However, in the stage (iii), since a catalystis given, a chemical reaction will progress; NO₂ will be removed and NH₃will be consumed. Consequently, absorptions of NO₂ and NH₃ start todecrease, and absorption of newly generated H₂O will be observedinstead. When more NH₃ is added, the absorption of NO will disappearcompletely and absorptions of NH₃ added superfluously and newlygenerated H₂O will increase.

Here, the use of the formula (13) enables the concentration of NO₂ to bemeasured quantitatively in the stage (iii). That is, as a large quantityof NH₃ is being added gradually, absorption of NO₂ will decrease, andabsorptions of NH₃ added superfluously and newly generated H₂O willappear. The concentration of NO₂ can be calculated with the formula(1.3) by measuring the quantity of NH₃ that is added up to any one ofthe following points: a point at which the absorption of N₂O becomeszero, a point at which the absorption of superfluous NH₃ starts toappear; and a point at which the absorption intensity of H₂O starts totake a constant value after it increased.

Since for the concentration of NH₃, only the quantity of addition shouldbe measured, measurement can be done correctly. In Embodiment 3-5, whenthe LiNbO₃ bulk crystal 341 of a length of 10 mm was used, the minimumdetection concentration of NO₂ was 1 ppm at 100 Torr. When the waveguideof a length of 10 mm was used, the minimum detection concentration ofNO₂ was able to be reduced to the order of 10 ppb.

The detection of NO gas is also done conveniently using the formula(12). The concentration of NO can be calculated by measuring thequantity of NH₃ that is added to any one of the following points: apoint at which the absorption of NO becomes zero when NH₃ and O₂ arebeing added to the gas removal tube 346 gradually, a point at which theabsorption of superfluous NH₃ starts to appear; and a point at which theabsorption intensity of H₂O starts to take a constant value after itincreased (here, the absorption of O₂ is not observed). Note howeverthat, since the wavelength of overtone absorption of NO and thewavelength of anti-symmetry stretching vibration absorption of H₂O arevery close to each other, totally symmetry stretching vibrationabsorption of H₂O and the absorption of NH₃ will mainly be employed. Theminimum detection concentration of NO gas was almost in the same levelas NO₂ gas.

In addition, since it is only necessary to prepare a single period Λ forthe LiNbO₃ bulk crystal 341 in Embodiment 3-5, the measurement isextremely simple and quick. Moreover, if what is required is to checkthe existence of NO and NO₂ gases, the measurement will become simplerand quicker, because it is only necessary to check the existence of aabsorption peak and it is not necessary to measure the quantity of NH₃.

Embodiment 3-6

In wavelengths of 2-3 μm, when a gas meter for gases of NO_(x), CO₂, CO,etc. is constructed with a tunable laser light source in themid-infrared region, gas concentrations of multiple kinds of gases canbe measured with a single unit of light source. Here, given is adescription of how to detect simultaneously four kinds of gases: NO,NO₂, CO, and CO₂. The fundamental absorption wavelength, the wavenumber, the name of absorption, and the overtone absorption wavelengthof targeted gases are as follows.

CO₂: 4.257 μm, 2349 cm⁻¹, anti-symmetry stretching vibration,overtone=2.129 μm

CO: 4.666 μm, 2143cm⁻¹, stretching vibration, overtone=2.333 μm

NO: 5.330 μm, 1876 cm⁻¹, anti-symmetry stretching vibration,overtone=2.665 μm

NO₂: 6.180 μm, 1618 cm⁻¹; anti-symmetry stretching vibration,overtone=3.090 μm

H₂O: 2.662 μm, 3756 cm⁻¹, anti-symmetry stretching vibration

H—O: 2.734 μm, 3657 cm⁻¹, totally symmetry stretching vibration

NH₃: 2.904 μm, 3444 cm⁻¹, double degenerated vibration

NH₃: 2.997 μm, 3337 cm⁻¹, totally symmetry vibration

In this embodiment, gases are made to pass through the following threestages, where each gas is removed one by one, and the gas concentrationof each gas was measured. The configuration is the same as that ofEmbodiment 3-5 shown in FIG. 19.

(a) NO, NO₂, CO₂, and CO gases are introduced into a gas removal tubewithout giving a catalyst and a gas for removable.

(b) A catalyst and NH₃ and O₂ gases are given to the gases to remove NOand NO₂ gases.

(c) After NO and NO₂ gases were removed in the (b), O₂ gas is given toburn CO gas In the stage (a), since no chemical reaction progresses inthe gas removal tube 346, overtone absorption of NO, NO₂, CO₂, and COgases is observed in wavelengths of 2-3 μm.

When the gases enter the stage (b), NO and NO₂ gases are removed andNH₃, gas is consumed, and in response to it, absorption of these gasesstarts to decrease and absorption of newly generated H₂O will beobserved. When surplus NH₃ and O₂ gases are added, absorption of NO andNO₂ gases will disappear completely and absorption of surplus NH₃ gasand newly generated H₂O gas will increase (also in this stage,absorption of O₂ is not observed). In the stage (c), as CO gas iscombusted according to the following reaction formula (14), absorptionof CO₂ will increase.2CO+O₂→2CO₂   (14)

In the stage (b), total concentrations of NO and NO₂ can be measuredquantitatively. That is, when a large quantity of NH₃ and O₂ are beingadded, absorption of NO and NO₂ will decrease, and absorption of NH₃added superfluously and newly generated H₂O will appear. Totalconcentrations of NO and NO₂ contained in the gas removal tube can becalculated with the formulas (12) and (13), if measuring the quantity ofNH₃ that is added up to any one of three points: a point at which theabsorptions of NO and NO₂ become zero, a point at which the absorptionof surplus NH₃ starts to appear, and a point at which the absorptionintensity of H₂O starts to take a constant value after it increased. Inorder to find individual concentrations of NO and NO₂, what is necessaryis to conduct the procedure specified in Embodiment 3-5.

In the stage (c), the concentration of CO can be measured. That is,combustion of CO under the existence of O₂ yields CO. Therefore, theconcentration of CO contained in the gas removal tube can be calculatedwith the formula (12) by measuring the quantity of O₂ that is added upto either of two points: a point at which the absorption of COdisappears when O₂ are being added, and a point at which the absorptionintensity of CO starts to take a constant value after it increased.Since for the quantity of O₂, only the quantity of addition should bemeasured, measurement can be done correctly. In Embodiment 3-6, when theLiNbO₃ bulk crystal 341 of a bulk length of 10 mm was used, the minimumdetection concentration of NO₂ was 1 ppm at 100 Torr. When the waveguideof a length of 10 mm was used, the minimum detection concentration ofNO₂ was able to be reduced to the order of 10 ppb.

Embodiment 3-7

With the use of a laser light source for generating mid-infrared lightaccording to this invention, gases of NO_(x), CO₂, CO, etc. each ofwhich has absorption in the wavelength band of 2-3 μm can be detected bya remote operation. In Embodiment 3-7, the two-wavelength differentialabsorption LIDAR (for example, see Non-patent document 11) was used todetect environmental gases. The two-wavelength differential absorptionLIDAR uses an absorption wavelength and a non-absorption wavelength of ameasured gas. Since a LIDAR signal of the absorption wavelength sufferslarger attenuation than that of the non-absorption wavelength, theconcentration of a gas molecule can be measured using a signaldifference between the two wavelengths.

In Embodiment 3-7, four kinds of gases, NO, NO₂, CO, and CO₂, aredetected by the two-wavelength differential absorption LIDAR. Thefundamental absorption wavelength, the wave number, the name ofabsorption, and the overtone absorption wavelength of each gas are asfollows.

CO₂: 4.257 μm, 2349 cm⁻¹, anti-symmetry stretching vibration,overtone=2.129 μm

CO: 4.666 μm, 2143cm⁻¹, stretching vibration, overtone=2.333 μm

NO: 5.330 μm, 1876 cm⁻¹, anti-symmetry stretching vibration,overtone=2.665 μm

NO₂: 6.180 μm, 1618 cm⁻¹, anti-symmetry stretching vibration,overtone=3.090 μm

In the measurement, it is required to perform measurement of twowavelengths at as close time points as possible in order to obtainaccurate data. Since the laser light source according to this inventioncan find two targeted wavelengths instantaneously and it is necessary toprepare only a single period Λ for LiNbO₃ crystal, four kinds of gasesin the wavelength band of 2-3 μm can be measured with an extremerapidity.

FIG. 20 shows a measurement system of a two-wavelength differentialabsorption LIDAR. A two-wavelength differential absorption LIDAR 360consists of a laser beam emission unit 360A and a laser beam detectionunit 360B. A laser light source included in the laser beam emission unit360A uses a LiNbO₃ crystal waveguide of a device length of 10 mm. Aperiod Λ of the periodically poled structure is specified as Λ=26 μm.The wavelength of the semiconductor laser 330 is specified in the0.91-μm wavelength band and the wavelength of the semiconductor laser331 is specified to be tunable between 1.28 μm and 1.46 μm wavelength.The laser beam emission unit 360A outputs mid-infrared light of awavelength of 2-3 μm from a laser exist window 361 under suitabletemperature adjustment.

Mid-infrared light 364 is emitted toward a detection gas 366. Scatteredlight 365 (Rayleigh scattering and Mie scattering) from the detectiongas 366 is received by a reflector 362 inside the laser beam detectionunit 360B. The focused beam is detected with a detector 363 that is aHgCdTe detector.

In the measurement, a non-absorption wavelength is set on 2-10 nm lowwavelength side from the overtone absorption wavelength of the detectiongas. The higher the intensity of the generated mid-infrared light, thelonger the detectable length grows. Because of this, the intensity ofthe mid-infrared light is set to a high power of 10 mW. When theaforesaid four gases are diffused to a concentration of 1 ppm in a spacethree meters away from the detector (a spherical space of a diameter ofone meter or more), absorption of all gases can be observed. If the gasconcentration is increased to 10 ppm, the gases can be detected in aspace to be measured ten meters away from the detector.

Embodiment 3-8

The laser light source for generating mid-infrared light according tothis invention is also useful to detect pesticides remaining inagricultural products. CN group and NO₂ group contained in pesticidesare typical examples of especially harmful functional groups. If theseare detected successfully, a degree of the concentration of residualpesticides can be known. CN group and NO₂ group are included infenpropathrin of the pyrethroid pesticide and1-naphthyl-N-methylcarbamate of the carbamate pesticide. Absorptionwavelengths are 4.44 μm for CN group (2250 cm−1, stretching vibration)and 6.15 μm for NO₂ group (1625 cm⁻¹, stretching vibration).

FIG. 21 shows a measurement system of a residual pesticide measuringinstrument. A residual pesticide measuring instrument 380 consists of alaser beam emission unit 380A and a laser beam detection unit 380B. Bymeans of optical fibers 381, 382 mounted on ends of the two units, abeam is emitted to an object under measurement consisting ofagricultural products and its scattered beam is detected by the laserbeam detection unit 380B. A HgCdTe detector and a PbSe detector are usedfor detectors installed inside the laser beam detection unit 380B. Thelaser light sources included in the laser beam emission unit 380A isunder appropriate temperature adjustment, and uses the LiNbO₃ crystalwaveguide that has a length of 10 mm and the periodically poledstructure with a period Λ=26 μm. The wavelength of one of thesemiconductor lasers was specified to be in the 0.91-μm wavelength band,and the other semiconductor laser 311 was specified to be tunable in awavelength band of 1.3-1.65 μm.

Fenpropathrin and 1-naphthyl-N-methylcarbamate are applied on the skinof an apple under test (concentration of 1%), and mid-infrared light ofa 10-mW power is irradiated onto this. As a result, overtone absorptionof CN group at a wavelength 2.22 μm and overtone absorption of NO₂ groupat a wavelength 3.08 μm were able to be observed sufficiently. TheEmbodiment 3-8 concludes that the existence of a plurality of functionalgroups can also be recognized with a LiNbO₃ crystal having a singleperiod Λ in detecting residual pesticides.

Note that, if a functional group to be detected is only NO, group, thisembodiment can also exhibit another advantage. That is, if setting theperiod of a LiNbO₃ crystal waveguide as Λ=27 μm (a period Λ=26 [μm maybe set, but a period Λ=27 μm is used for discussion in order to show themagnitude of the effect), the wavelength stability of the bothsemiconductor lasers used will improve when the absorption wavelength ofthe sample under test exists in a range of slightly larger than 3.0 μm,as described in Embodiment 3-4. So, even when using an optical systemsuch that a reflective film on the facet of the semiconductor laser andthe fiber Bragg grating of the optical fiber are removed, sufficientovertone absorption of NO₂ gas can be observed (incidentally, thiseffect can also be observed in the aforesaid detection of NO₂ gassimilarly).

Fourth Embodiment

FIG. 22 shows a laser light source for generating a wavelength of anoxygen-absorption line according to one embodiment of this invention.The laser light source for generating a wavelength of the oxygenabsorption line comprises: a distributed feedback semiconductor lasermodule 401 that oscillates laser light of a wavelength twice awavelength of one absorption line selected from oxygen absorption linesexisting at wavelengths of 759 nm to 768 nm; an optical waveguide 403having a second-order nonlinear optical effect; a polarizationmaintaining fiber 402 for connecting the semiconductor laser module 401and one end of the optical waveguide 403 having the second-ordernonlinear optical effect.

Since unlike the former examples, the semiconductor laser oscillates inthe wavelength band of 1518-1536 nm that is twice the wavelength band of759-768 nm, an indium phosphide system material is used for thesemiconductor laser. It is known that devices based on indium phosphidehardly suffer so-called sudden death as compared to devices based ongallium arsenide, and the reliability over device life is high.Moreover, the wavelengths of 1518 to 1536 nm belong to the S-band andthe C-band in the communication wavelength bands, so the manufacture ofDFB lasers is technically easy, thanks to recent development in theoptical communication field. Furthermore, a device as high-power as 40mW can be produced.

In the semiconductor laser of the indium phosphide system, changing thetemperature of a device or its injection current can vary thewavelength, and adopting a DFB type structure can perform stablewavelength scanning without mode jump. The laser light source convertslaser light of a wavelength 1518-1536 nm to light of, a wavelength759-768 nm and outputted it using the second overtone generation basedon the second-order nonlinear optical effect.

Here, the second-order nonlinear optical effect will be described. Thenonlinear optical effect is an effect that occurs in a matter becauseelectric polarization P in the substance has the high-order term of E²and E³ in addition to a term that is proportional to the electric fieldE of light as follows.P=χ ⁽¹⁾ E+χ ⁽²⁾ E ²+χ⁽³⁾ E ³+  (15)Especially, the second term is responsible for an effect that occursstrongly in a substance that lacks centro symmetry, yielding thefollowing effects, representing three lights having different angularfrequencies (ω₁, ω₂, and ω₃ that satisfy a relationship of (ω₁+ω₂=ω₃.

1) When light of ω₁ and light of ω₂ are inputted, light of ω₃ isgenerated (sum frequency generation).

2) In the case where ω₁ and ω₂ are the same angular frequency at thetime of sum frequency generation, a second overtone is generated.

3) When light of ω₁ and light of ω₃ are inputted, light of ω₂ (=ω₃−ω₁)is generated (difference frequency generation). That is, the wavelengthof an input laser beam can be converted to another wavelength.

The efficient wavelength converter has been realized by reversingpolarization of a second-order nonlinear optical material periodically.This structure is such that an influence of refractive index dispersiondue to a material is solved by reversing the polarization periodicallyto match phases of input light and converted light in a quasi manner. Asan example using this principle, there is known a wavelength convertersuch that polarization of, for example, lithium niobium oxide that is asecond-order nonlinear optical material is reversed periodically and awaveguide is formed therein by proton exchange (see Non-patent document12). It has been demonstrated that a lithium niobium oxide opticalwaveguide having such a periodically poled structure can generate asecond overtone whose energy reaches 90% or more of that of thefundamental light.

The optical waveguide having such the second-order nonlinear opticaleffect involves a life-related problem that the efficiency of the secondovertone generation decreases by a photorefractive effect. Since thisproblem does not occur with light of wavelengths of 1518 nm to 1536 nm,it occurs depending on the light intensity of wavelengths of 759 nm to768 nm that is a second overtone wave. However, it is known that theefficiency decrease can be avoided by increasing the temperature of anoptical waveguide having the second-order nonlinear optical effect from50° C. to about 100° C. or by using a second-order nonlinear opticalmaterial to which zinc or magnesium was doped (for example, seeNon-patent document 13); therefore, it is easy to obtain a long-lifeoptical waveguide.

Optical waveguides having such the second-order nonlinear optical effectexhibit the effect strongly to light that is polarized in a specificdirection with reference to a crystal orientation. For example, it isthe z-axis direction in lithium niobium oxide. The semiconductor laseroscillates with a certain polarization with reference to its substrate.Therefore, when the semiconductor laser module 401 and the opticalwaveguide 403 having the second-order nonlinear optical effect areconnected using an optical fiber, it is preferable to use thepolarization maintaining fiber 402 in order to suppress variation in thedirection of polarization of incident light to the optical waveguide.Incidentally, if the semiconductor laser module 401 is connected usingan optical fiber that is not a polarization maintaining fiber and apolarization controlling element is intercalated in the optical fiber,second overtone generation is possible. However, it is difficult togenerate a second overtone stably in a long period because polarizationin the optical fiber may fluctuate due to a change in externalenvironments, such as temperature.

FIG. 23 shows a laser light source equipped with a lens and a filter foroutput. That is, in addition to the laser light source in FIG. 22, theother end of an optical waveguide 413 having the second-order nonlinearoptical effect is equipped with a lens for collimating an emitted beamand a filter that allows beams of wavelengths of 759 nm to 768 nm topass through but does not allow beams of wavelengths of 1518 nm to 1536nm among the emitted beams to pass through. Thus, in the wavelengths of759 nm and 768 nm that are oxygen absorption lines, a beam forperforming stable wavelength scanning without mode jump can beextracted.

FIG. 24 shows a laser light source equipped with an optical fiber foroutput. In place of the embodiment in FIG. 23, an optical fiber 424 isconnected to the other end of an optical waveguide 423 having thesecond-order nonlinear optical effect. If the optical fiber 424 is of astructure capable of guiding light of wavelengths of 768 nm to 759 nm ina single mode, only light of wavelengths of 759 nm to 768 nm that arethe oxygen absorption lines can be taken out just by adding the opticalfiber 424 a slightest bend. This is because light of wavelengths of 1518nm to 1536 nm propagates in the optical fiber as a wide mode, andconsequently, if there is a part that suffers a slightest bend, suchlight is scattered in that part and attenuated in the optical fiber 424.

As described above, it becomes possible to provide a high-power,long-life laser light source that can output a laser beam of wavelengthsof 759 nm to 768 nm that are the oxygen absorption lines using secondovertone generation based on the second-order nonlinear optical effectof the optical waveguide, and perform stable wavelength scanning withoutmode jump.

Embodiment 4-1

FIG. 25 shows a laser light source according to an embodiment 4-1. Thelaser light source according to the embodiment 4-1 comprises: adistributed feedback semiconductor laser module 431 for oscillating alaser beam; an optical waveguide 433 having the second-order nonlinearoptical effect; and a polarization maintaining fiber 432 that connectsthe semiconductor laser module 431 and one end of the optical waveguidehaving the second-order nonlinear optical effect. At the other end 433 bof the optical waveguide 433 having the second-order nonlinear opticaleffect, a lens 435 for collimating an emitted beam and a filter 436 thatdoes not allow a beam near 1526 nm to pass through but allows a beamnear 763 nm to pass through among emitted beams.

The semiconductor laser module 431 oscillates a laser beam near 1526.08nm that is twice the 763.04-nm wavelength that is one of oxygenabsorption lines and is emitted from the polarization maintaining fiber432. The semiconductor laser module 431 has an internal Peltier device(not illustrated), which enables the temperature of the device to bevaried. Moreover, the semiconductor laser module has an internalisolator (not illustrated), which prevents a reflected beam from a facetof the optical waveguide etc. from causing an adverse effect on laseroscillation.

For a waveguide 433 having the second-order nonlinear optical effect,the periodically poled structure is formed on a lithium niobium oxidesubstrate. The formation of the waveguide is done using a methodaccording to the fifth embodiment or an annealed proton exchange method.A coating that is non-reflective to a wavelength of 1526 nm is formed onone end 433 a of the optical waveguide 433. Moreover, a coating thatbecomes non-reflective to a wavelength of 763 nm is formed on the otherend 433 b of the optical waveguide 433. Under the optical waveguide 433,disposed is a Peltier device that keeps the temperature of the opticalwaveguide 433 at 90° C. so that the second overtone generation maybecome most efficient at the 1526.08-nm wavelength of the incident beamon the optical waveguide 433.

When the semiconductor laser module 431 was set at 25° C. and operatedat the 1526.08-nm wavelength to deliver an output of 30 mW, light of awavelength 763.04 nm and an output of 5 mW is observed as an output beam437. The output beam 437 was observed while the temperature of thesemiconductor laser module 431 was varied from 24° C. to 26° C.continuously. The wavelength varied from 762.99 nm to 763.09 nmcontinuously, and any phenomenon like mode jump was not observed. Thelight intensity of the output beam 437 varied from 4.7 mW to 5.0 mW,showing a stable operation. This operation was performed continuouslythrough one year, and neither decrease in the output nor mode jump wasobserved.

Embodiment 4-2

FIG. 26 shows a laser light source according to an embodiment 4-2. Thelaser light source according to the embodiment 4-2 comprises: thedistributed feedback semiconductor laser module 401 that oscillates alaser beam; an optical waveguide 445 having the second-order nonlinearoptical effect; the polarization maintaining fiber 402 for connecting asemiconductor laser module 441 and one end 445 a of the opticalwaveguide 445 having the second-order nonlinear optical effect; and anoptical connector. An optical fiber 447 is connected to the other end445 b of the optical waveguide 445 having the second-order nonlinearoptical effect, and a lens 449 for collimating an emitted beam isdisposed near the optical fiber 447.

For the semiconductor laser module 441, the same module as that of theembodiment 4-1 was used. For the waveguide 445 having the second-ordernonlinear optical effect, the periodically poled structure is formed ona Zn-doped lithium niobium oxide substrate, and the waveguide is formedusing a method according to the fifth embodiment or the annealed protonexchange method. A coating that is non-reflective to the 1526-nmwavelength is formed on one end 445 a of the optical waveguide 445, towhich a polarization maintaining fiber 444 that guides a single mode tolight near the 1526-nm wavelength is connected. Moreover, a coating thatis non-reflective to the 763-nm wavelength is formed on the other end445 b of the optical waveguide 445, to which the polarizationmaintaining fiber 447 that guides a single mode to light near the 763-nmwavelength is connected.

Under the optical waveguide 445, disposed is a Peltier device 446 fortemperature control, which keeps the temperature of the opticalwaveguide 445 at 25.0° C. so that the second overtone generation maybecome most efficient at the 1526.08-nm wavelength of the incident lighton the optical waveguide 445. An optical fiber 442 and the optical fiber444 are connected with a connector 443, and an optical output of theoptical fiber 447 is collimated into a parallel beam with a lens 448.

When the semiconductor laser module 441 was set at 25° C. and operatedat the 1526.08-nm wavelength delivering an output of 30 mW, light of awavelength 763.04 nm and an output of 7 mW was observed as an outputbeam 449. The output beam 449 was observed while the temperature of thesemiconductor laser module was being varied from 24° C. to 26° C.continuously and the temperature of the optical waveguide 445 was beingvaried from 24° C. to 26° C. continuously by the Peltier device 446. Thewavelength varied from 762.99 nm to 763.09 nm continuously, the lightintensity of the output beam 449 varied from 6.9 mW to 7.0 mW, showingan extremely stable operation.

At this time, light of the 1526-nm wavelength that passed through thelaser light source without being converted into the second overtone wasbelow an observation limit in the output beam 449. This is because lightin the vicinity of 1526 nm propagates as a wider mode in the opticalfiber 447, and if there is a part that suffers a slightest bend, thelight is scattered at that part and attenuates in the optical fiber 447.Incidentally, a filter for removing the 1526-nm wavelength maybeinstalled downstream the lens 448 for safety's sake. Although thepolarization maintaining fiber was connected using the connector 443 inthe embodiment 4-2, it goes without saying that connection may be doneby fusion splice.

In this embodiment, paying attention to 763.04 nm that is one of oxygenabsorption lines, a semiconductor laser is selected and the laser lightsource is constructed with this laser. Alternatively, in order togenerate other absorption line existing between 759 nm and 768 nm, forexample 760.4 nm, the 1520.8-nm wavelength that is twice the 760.4-nmwavelength may be selected.

Although in this embodiment, a waveguide having the periodically poledstructure was used for the optical waveguide having the second-ordernonlinear optical effect, the same effect can be obtained using otherphase matching method. Moreover, for the substrate, lithium niobiumoxide or Zn-doped lithium niobium oxide was used. However, the sameeffect can be obtained even if using a mixed crystal of lithium niobiumoxide and lithium tantalum oxide, the mixed crystal to which a minutequantity of an element is doped, or other second-order nonlinear opticalmaterial. Furthermore, although the method according to the fifthembodiment or the annealed proton exchange method was used as a methodfor manufacturing a waveguide, naturally the same effect can be obtainedeven with the use of a metal diffused waveguide, such as Ti diffusion, aridge waveguide, an embedded waveguide, or the like.

It is needless to say that a waveguide structure may be altered for bothends and their neighborhoods of the optical waveguide having thesecond-order nonlinear optical effect so that it becomes easy to couplethe beam to optical fibers to be connected to the respective facets, orso that a shape of the beam when being emitted to space becomes optimal.Moreover, although the isolator was built in the semiconductor lasermodule, the reflected return light may be prevented by providinganti-reflection coatings on facets of the optical waveguide having thesecond-order nonlinear optical effect, cutting aslant the opticalwaveguide having the second-order nonlinear optical effect and arrangingoptical fibers or lenses accordingly, or combining these measures.

Fifth Embodiment

Next, a method for forming a waveguide in a nonlinear optical crystalwill be described. This embodiment uses a ridge-type waveguide using awafer-direct-bonded substrate. In the wafer-direct-bonding method, aLiNbO₃ substrate having the periodically poled structure matched to anoperating wavelength and a substrate whose surface has been treated arebonded directly at room temperature without an intermediate of anadhesive, and the substrates are subjected to annealing. For awaveguide, the periodically poled structure of the bonded substrate isground or made to be a thin film. Subsequently a ridge-type waveguide isformed on the bonded and thinned substrate using a dicing saw.

As a problem that the LiNbO₃ substrate has, improvement inoptical-damage resistance exists. The optical damage is a phenomenon inwhich light that is made to enter the waveguide excites carriers fromdefects existing in a crystal, subsequently the carriers are trapped inthe crystal, which induces refractive index change (photorefractiveeffect), and this change causes a shift in an operating wavelength.Since an operating wavelength band of the waveguide is as narrow as 1 nmdue to a LiNbO₃ substrate, in case an optical damage exists, the powerof output beam will decrease considerably, or even no power will beoutputted. In the waveguide element formed by applying the protonexchange method on a non-doped LiNbO₃ substrate, it is necessary to setthe operating temperature of the waveguide element at 100° C. or more inorder to realize sufficient optical damage resistance. However, there isa problem that, because of proton re-diffusion caused by this heating,long-term stability cannot be maintained. A waveguide element that isformed by applying the proton exchange method on a LiNbO₃ substrate towhich Mg or Zn was doped instead of a non-doped LiNbO₃ substrateexhibits a certain degree of improvement in the optical damageresistance. However, the waveguide device needs to be heated to 50° C.or more.

Here, using the wavelength conversion efficiency, power Pa of the sumfrequency light or the difference frequency light, is expressed by thefollowing formulaP=ηL ² P ₁ P ₂/100,and power Pb of the second overtone is expressed byPb=ηL ² P ₃ ²/100,where η is the efficiency per unit length (%/W/cm²), L is device length,and P₁, P₂, and P₃ are output beam powers of excitation lasers.

In this embodiment, the laser light source can operate at wavelengthsexcept for the wavelength band for optical communications, and deliverstable output of more than 10 mW by combining with a high-powersemiconductor laser of about 10-100W. Thus, the laser light source cangenerate a laser beam of an arbitrary wavelength in the wavelength bandof 450 nm to 5 μm where LiNbO₃ is transparent.

Embodiment 5-1

FIG. 27 shows a method for manufacturing a single-mode ridge-typewaveguide. A first substrate 501 is Z-cut Zn-doped LiNbO₃ substrate inwhich the periodically poled structure is formed in advance, and asecond substrate 502 is Z-cut Mg-doped LiNbO₃ substrate. Each of thesubstrates 501,502 is a 3-inch wafer whose both surfaces areoptical-polished and whose substrate thickness is 300 μm. The surfacesof the first substrate 501 and of the second substrate 502 were madehydrophilic by usual acid cleaning or alkali cleaning, and subsequentlythe substrates 501, 502 are superposed in clean atmosphere. Thesuperposed substrates 501,502 were put into an electric furnace andsubjected to diffusion bonding by heat-treating it at 400° C. for 3hours (first process). The bonded substrates 501,502 were void-free, andwhen being returned to room temperature, cracks etc. did not occur inthese substrates.

Next, the first substrate 501 was treated by grinding until itsthickness became 5 to 10 μm using grinding equipment whose turn tablefor grinding was under control in terms of flatness. After grindingprocess, the substrates 501, 502 are subjected to polishing to obtain aspecular polished surface (second process). The thickness of thesubstrates was measured with an optical thickness measuring instrument.Uniform thickness in the submicron range was obtained for almost thewhole surface, except for the periphery of the 3-inch wafer. Thus, athin film substrate suitable for formation of a waveguide was able to bemanufactured. Incidentally, an X-cut Zn-doped LiNbO₃ substrate may beused as the first substrate 501, and an X-cut Mg-doped LiNbO₃ substratemay be used as the second substrate 502.

A waveguide pattern was formed on the surface of the manufacturedthin-film substrata by a usual photolithographic process. Subsequently,the substrate was set in a dry etching apparatus, and the substratesurface was etched using CF₄ gas as an etching gas, whereby a core of awidth of 6-20 μm was formed to manufacture a ridge-type waveguide (thirdprocess). A waveguide element of the nonlinear optical crystal of alength of 10-60 mm can be obtained by cutting out a ridge-type waveguidefrom the wafer and polishing waveguide facets.

Embodiment 5-2

The first substrate 501 is a Z-cut Zn-doped LiNbO₃ substrate in whichthe periodically poled structure is formed in advance, and the secondsubstrate 502 is a Z-cut LiTaO₃ substrate. Each of the substrates501,502 is a 3-inch wafer whose both surfaces were optical-polished,having a thickness of 300 μm. The surfaces of the first substrate 501and of the second substrate 502 were made hydrophilic by usual acidcleaning or alkali cleaning, and subsequently the substrates 501, 502were superposed in clean atmosphere. The superposed substrates 501,502were put into an electric furnace and subjected to diffusion bonding byheat-treating it at 400° C. for 3 hours (first process). The bondedsubstrates 501, 502 were void-free, and when being returned to roomtemperature, cracks etc. did not occur in these substrates.

Next, the adhered substrates 501, 502 were treated by polishing usinggrinding equipment whose turn table for grinding was under control interms of flatness until the thickness of the first substrate 501 became6-10 μm. After the grinding, the substrates 501, 502 was subjected topolishing to obtain a specular polished surface (second process). Thethickness of the substrates was measured with an optical thicknessmeasuring instrument. Uniform thickness in the submicron range wasobtained for almost the whole surface, except for the periphery of the3-inch wafer. Thus, a thin film substrate suitable for formation of awaveguide was able to be manufactured. Incidentally, an X-cut Zn-dopedLiNbO₃ substrate may be used as the first substrate 501, and an X-cutLiTaO₃ substrate may be used as the second substrate 502.

A waveguide pattern was formed on the surface of the manufacturedthin-film substrata by a usual photolithographic process. Subsequently,the substrate was set in a dry etching apparatus, and the substratesurface was etched using CF₄ gas as an etching gas, whereby a core of awidth of 6-20 μm was formed to manufacture a ridge-type waveguide (thirdprocess). A waveguide element of the nonlinear optical crystal of alength of 10-60 mm can be obtained by cutting out a ridge-type waveguidefrom the wafer and polishing waveguide facets.

Embodiment 5-3

The first substrate 501 is a LiNbO₃ substrate in which the periodicallypoled structure is formed in advance, and the second substrate 502 is aquartz substrate. The thermal expansion coefficient of quartz in anin-plane direction perpendicular to Z-axis is 13.6×10⁻⁶/K, which isclose to the thermal expansion coefficient of LiNbO₃, and the refractiveindex of quartz is 1.53, which is smaller than the refractive index ofLiNbO₃, 2.1. Consequently, this combination is suitable for manufactureof a waveguide. By the same manufacture method as that of the embodiment5-1, a waveguide element of a nonlinear optical crystal can be obtained.

A Mg-doped LiNbO₃ substrate, a Sc-doped LiNbO₃ substrate, an In-dopedLiNbO₃ substrate, a LiTaO₃ substrate, a LiNb_(x)Ta_(1-x)O₃ substrate, aKNbO₃ substrate, a KTiNbO₃ substrate, etc. may be used as the firstsubstrate 501, instead of the Zn-doped LiNbO₃ substrate.

Embodiment 5-4

In order to form a waveguide of an embodiment 5-4, precision grinding bya dicing saw is performed on the substrate that is manufactured up tothe second process of the embodiment 5-1. That is the ground substrateis set in a dicing saw, and a ridge-type waveguide having a core of awidth of 6 μm is manufactured by precision machining using a diamondblade whose particles are 4 μm in diameter (third process). A waveguideelement made of the nonlinear optical crystal of a length of 10-60 mmcan be obtained by cutting out a ridge-type waveguide and opticalpolishing the facets of the waveguide. Incidentally, the substratemanufactured in the embodiment 5-2 or in the embodiment 5-3 may be used.

According to this embodiment, the accuracy in refractive indexmeasurement at the sodium D line can be improved by about two orders ofmagnitude compared to the present state. Therefore, quality control offoods or medicines can be improved largely, and in addition the safetycan be improved largely by increasing monitoring accuracy for foreignmatters and inclusion of poisons. Moreover, regarding a substance whoserelationship between the refractive index and the density is known, itbecomes possible to obtain the density from the measurement of therefractive index, and the accuracy in this density measurement is alsoenhanced remarkably.

Moreover, according to this embodiment, a compact and economical lasermicroscope, flow cytometer, etc. can be realized by adopting anenergy-efficient, compact, and low-consumption laser light source.

Furthermore, the laser light source for generating mid-infrared lightaccording to this embodiment can detect environmental gases accurately,and can be applied to a measurement instrument for detecting pesticidesthat remain in agricultural products.

Even further, the laser light source can be used as a laser light sourcethat is used for an oxymeter and outputs a laser beam of wavelengths of759 nm to 768 nm that are oxygen absorption lines.

1-36. (canceled)
 37. A laser light source comprising a first laser forgenerating a laser beam of a wavelength λ₁, a second laser forgenerating a laser beam of a wavelength λ₂, and a nonlinear opticalcrystal that uses the laser beam of the wavelength λ₁ and the laser beamof the wavelength λ₂ as inputs and outputs a coherent beam having awavelength λ₃ of a sum frequency that satisfies a relationship of1/λ₁+1/λ₂=1/λ₃, and wherein the wavelength λ₃ of a sum frequency is awavelength of 589.3±2 nm that is equivalent to the sodium D line. 38.The laser light source according to claim 37, wherein, representingrefractive indices of the wavelengths λ₁, λ₂, and λ₃ by n₁, n₂, and n₃,respectively, the nonlinear optical crystal has a periodically poledstructure of a period Λ that satisfies 2πn₃/λ₃=2λn₁/λ₁+2πn₂/λ₂+2πn₂/Λ.39. The laser light source according to claim 38, wherein, the nonlinearoptical crystal has a waveguide structure.
 40. The laser light sourceaccording to claim 37, wherein, the wavelength λ₁ is 976±10 nm and thewavelength λ₂ is 1485±20 nm.
 41. The laser light source according toclaim 37, wherein, the wavelength λ₁ is 1064±10 nm and the wavelength λ₂is 1320±20 nm.
 42. The laser light source according to claim 37,wherein, the wavelength λ₁ is 940±10 nm and the wavelength λ₂ is 1565±35nm.
 43. The laser light source according to claim 40, wherein the secondlaser for outputting a wavelength λ₂=1485±20 nm is a DFB laser.
 44. Thelaser light source according to claim 41, wherein the second laser foroutputting a wavelength λ₃=1320±20 nm is a DFB laser.
 45. The laserlight source according to claim 42, wherein the second laser foroutputting a wavelength λ₂=1565±35 nm is a DFB laser.
 46. The laserlight source according to claim 37, further comprising: two polarizationmaintaining fibers coupled to outputs of the first and second lasers,respectively; and a multiplexer for multiplexing outputs of the twopolarization maintaining fibers and coupling a multiplexed output to thenonlinear optical crystal.
 47. The laser light source according to claim46, wherein the first and second excitation lasers are semiconductorlasers, and at least one of the two polarization maintaining fibers hasa fiber Bragg grating.
 48. The laser light source according to claim 47,wherein at least one of the first and second lasers has a first facetthat is coupled to the polarization maintaining fiber and a second facetopposite to the first facet, the first facet being specified to have areflectance of 2% or less and the second facet being specified to have areflectance of 90% or more.
 49. A laser light source comprising a firstlaser for generating a laser beam of a wavelength λ₁, a second laser forgenerating a laser beam of a wavelength λ₂, and a nonlinear opticalcrystal that uses the laser beam of the wavelength λ₁ and the laser beamof the wavelength λ₂ as inputs and outputs a coherent beam having awavelength λ₃ of a sum frequency that satisfies a relationship of1/λ₁+1/λ₂=1/λ₃, and wherein the wavelength λ₁ is 940±10 nm, thewavelength λ₂ is 1320±20 nm, and the wavelength λ₃ of the sum frequencyis a wavelength of 546.1±5.0 nm corresponding to a yellow range.
 50. Thelaser light source according to claim 49, wherein, representingrefractive indices at the wavelengths λ₁, λ₂, and λ₃ by n₁, n₂, and n₃,respectively, the nonlinear optical crystal has a periodically poledstructure of a period Λ that satisfies 2πn₃/λ₃=2πn₁/λ₁+2πn₂/λ₂+2πn₂/Λ.51. The laser light source according to claim 50, wherein the nonlinearoptical crystal has a waveguide structure.
 52. The laser light sourceaccording to claim 49, wherein the second laser is a DFB laser.
 53. Thelaser light source according to claim 49, further comprising: twopolarization maintaining fibers coupled to outputs of the first andsecond lasers, respectively; and a multiplexer for multiplexing outputsof the two polarization maintaining fibers and coupling a multiplexedoutput to the nonlinear optical crystal.
 54. The laser light sourceaccording to claim 53, wherein the first and second excitation lasersare semiconductor lasers, and at least one of the two polarizationmaintaining fibers has a fiber Bragg grating.
 55. The laser light sourceaccording to claim 54, wherein at least one of the first and secondlasers has a first facet that is coupled to the polarization maintainingfiber and a second facet opposite to the first facet, the first facetbeing specified to have a reflectance of 2% or less and the second facetbeing specified to have a reflectance of 90% or more.
 56. A laser lightsource comprising a first laser for generating a laser beam of awavelength λ₁, a second laser for generating a laser beam of awavelength λ₂, and a nonlinear optical crystal that uses the laser beamof the wavelength λ₁ and the laser beam of the wavelength λ₂ as inputsand outputs a coherent beam having a wavelength λ₃ of a sum frequencythat satisfies a relationship of 1/λ₁+1/λ₂=1/λ₃, and wherein thewavelength λ₁ is 980±10 nm, the wavelength λ₂ is 1320±20 nm, and thewavelength λ₃ of the sum frequency is a wavelength of 560.0±5.0 nmcorresponding to a yellow range.
 57. The laser light source according toclaim 56, wherein, representing refractive indices at the wavelengthsλ₁, λ₂, and λ₃ by n₁, n₂, and n₃, respectively, the nonlinear opticalcrystal has a periodically poled structure of a period Λ that satisfies2πn₃/λ₃=2πn₁/λ₁+2πn₂/λ₂+2πn₂/Λ.
 58. The laser light source according toclaim 57, wherein the nonlinear optical crystal has a waveguidestructure.
 59. The laser light source according to claim 56, wherein thesecond laser is a DFB laser.
 60. The laser light source according toclaim 56, further comprising: two polarization maintaining fiberscoupled to outputs of the first and second lasers, respectively; and amultiplexer for multiplexing outputs of the two polarization maintainingfibers and coupling a multiplexed output to the nonlinear opticalcrystal.
 61. The laser light source according to claim 60, wherein thefirst and second excitation lasers are semiconductor lasers, and atleast one of the two polarization maintaining fibers has a fiber Bragggrating.
 62. The laser light source according to claim 61, wherein atleast one of the first and second lasers has a first facet that iscoupled to the polarization maintaining fiber and a second facetopposite to the first facet, the first facet being specified to have areflectance of 2% or less and the second facet being specified to have areflectance of 90% or more.
 63. A laser light source comprising a firstlaser for generating a laser beam of a wavelength λ₁, a second laser forgenerating a laser beam of a wavelength λ₂, and a nonlinear opticalcrystal that uses the laser beam of the wavelength λ₁ and the laser beamof a wavelength λ₂ as inputs and outputs a coherent beam having awavelength λ₃ of a sum frequency that satisfies a relationship of1/λ₁+1/λ₂=1/λ₃, and wherein the wavelength λ₁ is 1064±10 nm, thewavelength λ₂ is 1320±20 nm, and the wavelength λ₃ of the sum frequencyis a wavelength of 585.0±5.0 nm corresponding to a yellow range.
 64. Thelaser light source according to claims 63, wherein, representingrefractive indices at the wavelengths λ₁, λ₂, and λ₃ by n₁, n₂, and n₃,respectively, the nonlinear optical crystal has a periodically poledstructure of a period Λ that satisfies 2πn₃/λ₃=2πn₁/λ₁+2πn₂/λ₂+2πn₂/Λ.65. The laser light source according to claim 64, wherein the nonlinearoptical crystal has a waveguide structure.
 66. The laser light sourceaccording to claim 63, wherein the second laser is a DFB laser.
 67. Thelaser light source according to claim 63, further comprising: twopolarization maintaining fibers coupled to outputs of the first andsecond lasers, respectively; and a multiplexer for multiplexing outputsof the two polarization maintaining fibers and coupling a multiplexedoutput to the nonlinear optical crystal.
 68. The laser light sourceaccording to claim 67, wherein the first and second excitation lasersare semiconductor lasers, and at least one of the two polarizationmaintaining fibers has a fiber Bragg grating.
 69. The laser light sourceaccording to claim 68, wherein at least one of the first and secondlasers has a first facet that is coupled to the polarization maintainingfiber and a second facet opposite to the first facet, the first facetbeing specified to have a reflectance of 2% or less and the second facetbeing specified to have a reflectance of 90% or more.
 70. A laser lightsource comprising a first laser for generating a laser beam of awavelength λ₁, a second laser for generating a laser beam of awavelength λ₂, and a nonlinear optical crystal that uses the laser beamof the wavelength λ₁ and the laser beam of the wavelength λ₂ as inputsand outputs a coherent beam having a wavelength λ₃ of a sum frequencythat satisfies a relationship of 1/λ₁+1/λ₂=1/λ₃, and wherein thewavelength λ₁ is 940±10 nm, the wavelength is λ₂ is 1550±30 nm, and thewavelength λ₃ of the sum frequency is a wavelength of 585.0±5.0 nmcorresponding to a yellow range.
 71. The laser light source according toclaim 70, wherein, representing refractive indices at the wavelengthsλ₁, λ₂, and λ₃ by n₁, n₂, and n₃, respectively, the nonlinear opticalcrystal has a periodically poled structure of a period Λ that satisfies2πn₃/λ₃=2πn₁/λ₁+2πn₂/λ₂+2πn₂/Λ.
 72. The laser light source according toclaim 71, wherein the nonlinear optical crystal has a waveguidestructure.
 73. The laser light source according to claim 70, wherein thesecond laser is a DFB laser.
 74. The laser light source according toclaim 70, further comprising: two polarization maintaining fiberscoupled to outputs of the first and second lasers, respectively; and amultiplexer for multiplexing outputs of the two polarization maintainingfibers and coupling a multiplexed output to the nonlinear opticalcrystal.
 75. The laser light source according to claim 74, wherein thefirst and second excitation lasers are semiconductor lasers, and atleast one of the two polarization maintaining fibers has a fiber Bragggrating.
 76. The laser light source according to claim 75, wherein atleast one of the first and second lasers has a first facet that iscoupled to the polarization maintaining fiber and a second facetopposite to the first facet, the first facet being specified to have areflectance of 2% or less and the second facet being specified to have areflectance of 90% or more.